s390: fold execve_tail() into start_thread(), convert to generic sys_execve()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/arch/s390/include/asm/unistd.h b/arch/s390/include/asm/unistd.h
index 6756e78..9afacff 100644
--- a/arch/s390/include/asm/unistd.h
+++ b/arch/s390/include/asm/unistd.h
@@ -415,6 +415,7 @@
 #   define __ARCH_WANT_COMPAT_SYS_TIME
 #   define __ARCH_WANT_COMPAT_SYS_RT_SIGSUSPEND
 # endif
+#define __ARCH_WANT_SYS_EXECVE
 
 /*
  * "Conditional" syscalls