rt: automatically align .text by default - #692

Fixes #678

An alternative I considered is this:

PROVIDE(_stext = ADDR(.vector_table) + SIZEOF(.vector_table));
. = _stext;
.text :
{
  __stext = .;
  ...
}

This lets the linker align the section itself naturally, which is arguably more "idiomatic" (if such a concept can ever exist for linker scripts) but thi means _stext and __stext can have different values which is too much of a footgun.