[PATCH] selftests/nolibc: Fix up compile error for rv32

[Willy Tarreau]

Thomas, Zhangjin,

I've merged your latest patches in my branch 20230520-nolibc-rv32+stkp2,
which was rebased to integrate the updated commit messages and a few
missing s-o-b from mine. Please have a look:

   https://git.kernel.org/pub/scm/linux/kernel/git/wtarreau/nolibc.git

However, Thomas, I noticed something puzzling me. While I tested with
gcc-9.5 (that I have here along my toolchains) I found that it would
systematically fail:

  sysroot/x86/include/stackprotector.h:46:1: warning: 'no_stack_protector' attribute directive ignored [-Wattributes]
     46 | {
        | ^
  !!Stack smashing detected!!
  qemu: uncaught target signal 6 (Aborted) - core dumped
  0 test(s) passed.

The reason is that it doesn't support the attribute "no_stack_protector".
Upon closer investigation, I noticed that _start() on x86_64 doens't have
it, yet it works on more recent compilers! So I don't understand why it
works with more recent compilers.

I managed to avoid the crash by enclosing the __stack_chk_init() function
in a #pragma GCC optimize("-fno-stack-protector") while removing the
attribute (though Clang and more recent gcc use this attribute so we
shouldn't completely drop it either).

I consider this non-critical as we can expect that regtests are run with
a reasonably recent compiler version, but if in the long term we can find
a more reliable detection for this, it would be nice.

For example I found that gcc defines __SSP_ALL__ to 1 when
-fstack-protector is used, and 2 when -fstack-protector-all is used.
With clang, it's 1 and 3 respectively. Maybe we should use that and
drop NOLIBC_STACKPROTECTOR, that would be one less variable to deal
with: the code would automatically adapt to whatever cflags the user
sets on the compiler, which is generally better.

Regards,
Willy