binfmt: parse application stacksize/priority from ELF symbols

[anjiahao1]

Summary

• Allows an application's configured stack size and priority to be carried
  as absolute ELF symbols (nx_stacksize, nx_priority, and
  nx_uid/nx_gid/nx_mode under CONFIG_SCHED_USER_IDENTITY) and read
  back by the binary loader at exec time, instead of always falling back
  to CONFIG_ELF_STACKSIZE / SCHED_PRIORITY_DEFAULT.
• Adds libelf_findsymbol() (a name-based symbol lookup over the ELF
  symbol table) in libs/libc/elf/elf_symbols.c and declares it in
  include/nuttx/lib/elf.h; binfmt/elf.c uses it to populate
  struct binary_s (binp->stacksize/priority/uid/gid/mode),
  falling back to the existing defaults when a symbol is absent.
• cmake/nuttx_add_application.cmake emits the --defsym nx_* flags at
  link time.
• posix_spawnattr_init() no longer pre-fills attr->stacksize and
  attr->priority (left at 0); nxtask_spawn_exec() fills the parent
  priority and CONFIG_POSIX_SPAWN_DEFAULT_STACKSIZE when a caller does
  not set them. This stops the spawnattr defaults from overriding the
  values parsed from the ELF, so the embedded symbols actually drive the
  spawned task.
• Related NuttX Apps PR: Application.mk: emit application attribute as ELF symbols nuttx-apps#3641

Impact

• Is new feature added? Is existing feature changed? YES — new feature:
  application attributes are parsed from ELF symbols and used by the
  binary loader.
• Impact on user (will user need to adapt to change)? NO — the symbols
  are optional; when absent the loader keeps using the existing defaults
  (CONFIG_ELF_STACKSIZE, SCHED_PRIORITY_DEFAULT).
• Impact on build (will build process change)? YES (minor) — the ELF app
  link now emits --defsym nx_stacksize/nx_priority (and nx_uid/
  nx_gid/nx_mode under CONFIG_SCHED_USER_IDENTITY); handled in the
  companion apps PR via Application.mk.
• Impact on hardware (will arch(s) / board(s) / driver(s) change)? NO.
• Impact on documentation (is update required / provided)? NO (not yet).
• Impact on security (any sort of implications)? NO.
• Impact on compatibility (backward/forward/interoperability)? NO —
  posix_spawnattr_init() callers that relied on the old stacksize/
  priority defaults still get them filled in by nxtask_spawn_exec() /
  the binary loader, so observable defaults are unchanged; only apps that
  embed the nx_* symbols change behavior (they now honor their own
  configured values).
• Anything else to consider or add? The heapsize attribute from the
  original implementation is intentionally not included (it depends on
  the separate task-heap feature).

Testing

I confirm that changes are verified on local setup and works as intended:

• Build Host(s): Ubuntu, arm-none-eabi-gcc 12.2.1
• Target(s): qemu-armv7a:knsh (CONFIG_BUILD_KERNEL, CONFIG_ELF=y)
• Build: ./tools/configure.sh qemu-armv7a:knsh && make && make export
  (apps built separately and mounted via hostfs as /system).
• Run command: qemu-system-arm -semihosting -M virt -m 1024 -nographic -kernel ./nuttx

Testing logs before change (stock upstream/master nuttx; the getprime
app was built with --defsym nx_stacksize=8192 --defsym nx_priority=50,
but the symbol is not consulted, so the task uses the defaults — stack
4096, priority 100):

nsh> getprime 4 &
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK    USED FILLED COMMAND
    4     4     3 100 RR       Task      - Waiting  Semaphore 0000000000000000 0004048 0000776  19.1%  getprime 4

Testing logs after change (this PR; same getprime app — the ELF symbols
now drive the task: stack ≈ 8192, priority 50):

nsh> getprime 4 &
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK    USED FILLED COMMAND
    4     4     3  50 RR       Task      - Waiting  Semaphore 0000000000000000 0008144 0000792   9.7%  getprime 4

hello, ostest, and NSH itself continue to load and run; no regression
observed on knsh.

PR verification Self-Check

• This PR introduces only one functional change.
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines and Documentation (git commit title and message, coding standard, etc).
• My PR is still work in progress (not ready for review).
• My PR is ready for review and can be safely merged into a codebase.

[github-actions]

MemBrowse Memory Report

arduino-mega2560

• flash: .text +30 B (+0.0%, 67,248 B / 262,144 B, total: 26% used)

esp32-devkitc

• ROM: .flash.text +8 B (+0.0%, 122,716 B / 4,194,272 B, total: 3% used)
• irom0_0_seg: .flash.text +8 B (+0.0%, 87,084 B / 3,342,304 B, total: 3% used)

mirtoo

• kseg0_progmem: .text +20 B (+0.0%, 67,152 B / 131,072 B, total: 51% used)

qemu-armv8a

• Code: .text.posix_spawnattr_init -16 B, .text.task_spawn +36 B (+0.0%, 315,374 B)

qemu-intel64

• Code: .text +48 B (+0.0%, 8,655,445 B)

rx65n-rsk2mb

• ROM: .text +16 B (+0.0%, 85,656 B / 2,097,152 B, total: 4% used)

s698pm-dkit

• Code: .text +736 B (+0.2%, 358,096 B)

stm32-nucleo-f103rb

• flash: .text +20 B (+0.1%, 33,792 B / 131,072 B, total: 26% used)
  No memory changes detected for:
• hifive1-revb

[acassis]

@raiden00pl nucleo-f302r8/ihm07m1_b16 is overflowing again, maybe we need to remove this board, end of life for NuttX on it. :-(