Fix sysdeps/unix/sysv/linux/arm/libc-do-syscall.S warning.

Fix sysdeps/unix/sysv/linux/arm/libc-do-syscall.S warning.

This patch fixes a warning

../include/features.h:328:4: warning: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Wcpp]

seen when building for ARM.  This warning comes from libc-do-syscall.S
being built for nscd: the nscd build uses _FORTIFY_SOURCE, while .S
files aren't built with -O, and the combination produces a warning.
As _FORTIFY_SOURCE doesn't do anything for .S files, undefining it in
libc-do-syscall.S seems the simplest solution.

Tested for ARM.

	* sysdeps/unix/sysv/linux/arm/libc-do-syscall.S (_FORTIFY_SOURCE):
	Undefine.