tools/nolibc: export environ as a weak symbol on s390

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 on s390 both with environ inherited from
_start and extracted from envp.
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/arch-s390.h

@@ -159,6 +159,8 @@ struct sys_stat_struct {
 	_arg1;								\
 })
 
+char **environ __attribute__((weak));
+
 /* startup code */
 void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
 {
@@ -174,6 +176,8 @@ void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
 		"la	%r4,8(%r4)\n"		/* advance pointer */
 		"jnz	0b\n"			/* no -> test next pointer */
 						/* yes -> r4 now contains start of envp */
+		"larl	%r1,environ\n"
+		"stg	%r4,0(%r1)\n"
 
 		"aghi	%r15,-160\n"		/* allocate new stackframe */
 		"xc	0(8,%r15),0(%r15)\n"	/* clear backchain */