uml: fix a section warning

When compiling uml on x86_64:

  MODPOST vmlinux.o
WARNING: vmlinux.o (.__syscall_stub.2): unexpected non-allocatable section.
Did you forget to use "ax"/"aw" in a .S file?
Note that for example <linux/init.h> contains
section definitions for use in .S files.

Because modpost checks for missing SHF_ALLOC section flag.  So just add
it.
Signed-off-by: WANG Cong <amwang@redhat.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/sys-i386/stub.S

 #include "as-layout.h"
 
 	.globl syscall_stub
-.section .__syscall_stub, "x"
+.section .__syscall_stub, "ax"
 
 	.globl batch_syscall_stub
 batch_syscall_stub:

arch/um/sys-x86_64/stub.S

 #include "as-layout.h"
 
 	.globl syscall_stub
-.section .__syscall_stub, "x"
+.section .__syscall_stub, "ax"
 syscall_stub:
 	syscall
 	/* We don't have 64-bit constants, so this constructs the address