tools/nolibc: silence ppc64 compile warnings

tools/nolibc: silence ppc64 compile warnings

Silence the following warnings reported by the new -Wall -Wextra options
with pure assembly code.

    In file included from sysroot/powerpc/include/stdio.h:13,
                     from nolibc-test.c:13:
    sysroot/powerpc/include/arch.h: In function '_start':
    sysroot/powerpc/include/arch.h:192:32: warning: unused variable 'r2' [-Wunused-variable]
      192 |         register volatile long r2 __asm__ ("r2") = (void *)&TOC - (void *)_start;
          |                                ^~
    sysroot/powerpc/include/arch.h:187:97: warning: optimization may eliminate reads and/or writes to register variables [-Wvolatile-register-var]
      187 | void __attribute__((weak, noreturn, optimize("Os", "omit-frame-pointer"))) __no_stack_protector _start(void)
          |                                                                                                 ^~~~~~

Since only elfv2 ABI requires to save the TOC/GOT pointer to r2
register, when using elfv1 ABI, the old C code is simply ignored by the
compiler, but the compiler can not ignore the inline assembly code and
will introduce build failure or running segfaults. So, let's further
only add the new assembly code for elfv2 ABI with the checking of
_CALL_ELF == 2.

Link: https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.pdf
Link: https://www.llvm.org/devmtg/2014-04/PDFs/Talks/Euro-LLVM-2014-Weigand.pdf
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>