tools/nolibc: mark arg1 operand in __nolibc_syscall0() as write-only

__nolibc_syscall0() does not set the arg1 variable before passing it to
the asm block. This uninitialized variable read is undefined behavior.
Clang can miscompile this.

Mark the asm operand as write-only to fix this.

Fixes: 8e1930296f ("tools/nolibc: Add support for SPARC")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260703-nolibc-sparc-asm-v1-1-c7fe73e2e777@weissschuh.net

tools/include/nolibc/arch-sparc.h

@@ -45,7 +45,7 @@
 									      \
 	__asm__ volatile (                                                    \
 		_NOLIBC_SYSCALL                                               \
-		: "+r"(_arg1)                                                 \
+		: "=r"(_arg1)                                                 \
 		: "r"(_num)                                                   \
 		: "memory", "cc"                                              \
 	);                                                                    \