sparc: remove asm constraint

uClibc-ng don't build with gcc 9.1 [1] due to a new check that
"catch illegal asm constraint usage" [2].
gcc 9.1 print this error:
"invalid hard register usage between earlyclobber operand and input operand"
The asm constraint is present in uClibc since it support sparc (back in 2002)[3].
Note: There is no such constraint is Glibc counterpart code [4].
[1] https://gitlab.com/kubu93/toolchains-builder/-/jobs/205435757
[2] https://github.com/gcc-mirror/gcc/commit/b782636f28f5c378897c238081d28d7a4a6ca578
[3] https://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/commit/?id=3b6d086531102b6d09ce852feb1e370d5dca3ce9
[4]
+https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/sparc/sysdep.h;h=981b2a26b7a91093f821c97876
+e55bc4be2d9f8a;hb=HEAD

libc/sysdeps/linux/sparc/bits/syscalls.h

@@ -33,7 +33,7 @@
             register long __g1 __asm__("g1") = sys_num;     \
             LOAD_ARGS_##nr(args)                            \
             __asm__ __volatile__( __SYSCALL_STRING          \
-                : "=r" (__res), "=&r" (__o0)                \
+                : "=r" (__res), "=r" (__o0)                \
                 : "1" (__o0) ASM_ARGS_##nr, "r" (__g1)     \
                 : __SYSCALL_CLOBBERS );                                   \
         }                                                   \