link_warning fix

[Bruno Haible]

Ulrich Drepper writes:

> > I don't agree. We put a strange string "foobar\n#APP\n\t#" into the
> > section name. It is unreasonable to expect that gcc will scan user
> > given strings to see if they contain assembler directives.
> 
> Where is this done?  And if there is a place it is also the place to add
> the NO_APP marker.

The comment in include/libc-symbols.h says that the section attributes
that gcc emits are unwanted:

	.section	.gnu.warning.sigstack,"a",@progbits

The hack consists in adding the #APP and comment markers in the
section name:

	.section	.gnu.warning.sigstack
#APP
#,"a",@progbits

But gcc does not offer a 'post-section-statement-hook' where we could
silently emit a #NO_APP pseudo-instruction. So we have to do it
through  __asm__("").

Bruno