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.pdfSigned-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>

tools/include/nolibc/arch-powerpc.h

@@ -187,9 +187,17 @@
 void __attribute__((weak, noreturn, optimize("Os", "omit-frame-pointer"))) __no_stack_protector _start(void)
 {
 #ifdef __powerpc64__
-	/* On 64-bit PowerPC, save TOC/GOT pointer to r2 */
-	extern char TOC __asm__ (".TOC.");
-	register volatile long r2 __asm__ ("r2") = (void *)&TOC - (void *)_start;
+#if _CALL_ELF == 2
+	/* with -mabi=elfv2, save TOC/GOT pointer to r2
+	 * r12 is global entry pointer, we use it to compute TOC from r12
+	 * https://www.llvm.org/devmtg/2014-04/PDFs/Talks/Euro-LLVM-2014-Weigand.pdf
+	 * https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.pdf
+	 */
+	__asm__ volatile (
+		"addis  2, 12, .TOC. - _start@ha\n"
+		"addi   2,  2, .TOC. - _start@l\n"
+	);
+#endif /* _CALL_ELF == 2 */
 
 	__asm__ volatile (
 		"mr     3, 1\n"         /* save stack pointer to r3, as arg1 of _start_c */