tools/nolibc: export environ as a weak symbol on x86_64

The environ is retrieved from the _start code and is easy to store at
this moment. Let's declare the variable weak and store the value into
it. By not being static it will be visible to all units. By being weak,
if some programs already declared it, they will continue to be able to
use it. This was tested both with environ inherited from _start and
extracted from envp.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/arch-x86_64.h

@@ -178,6 +178,8 @@ struct sys_stat_struct {
 	_ret;                                                                 \
 })
 
+char **environ __attribute__((weak));
+
 /* startup code */
 /*
  * x86-64 System V ABI mandates:
@@ -191,6 +193,7 @@ void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
 		"pop %rdi\n"                // argc   (first arg, %rdi)
 		"mov %rsp, %rsi\n"          // argv[] (second arg, %rsi)
 		"lea 8(%rsi,%rdi,8),%rdx\n" // then a NULL then envp (third arg, %rdx)
+		"mov %rdx, environ\n"       // save environ
 		"xor %ebp, %ebp\n"          // zero the stack frame
 		"and $-16, %rsp\n"          // x86 ABI : esp must be 16-byte aligned before call
 		"call main\n"               // main() returns the status code, we'll exit with it.