tools/nolibc: add auxiliary vector retrieval for s390

In the _start block we now iterate over envp to find the auxiliary
vector after the NULL. The pointer is saved into an _auxv variable
that is marked as weak so that it's accessible from multiple units.
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

@@ -160,6 +160,7 @@ struct sys_stat_struct {
 })
 
 char **environ __attribute__((weak));
+const unsigned long *_auxv __attribute__((weak));
 
 /* startup code */
 void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
@@ -179,6 +180,15 @@ void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
 		"larl	%r1,environ\n"
 		"stg	%r4,0(%r1)\n"
 
+		/* search for auxv */
+		"lgr	%r5,%r4\n"		/* start at envp */
+		"1:\n"
+		"clg	%r0,0(%r5)\n"		/* entry zero? */
+		"la	%r5,8(%r5)\n"		/* advance pointer */
+		"jnz	1b\n"			/* no -> test next pointer */
+		"larl	%r1,_auxv\n"		/* yes -> store value in _auxv */
+		"stg	%r5,0(%r1)\n"
+
 		"aghi	%r15,-160\n"		/* allocate new stackframe */
 		"xc	0(8,%r15),0(%r15)\n"	/* clear backchain */
 		"brasl	%r14,main\n"		/* ret value of main is arg to exit */