Fix MIPS _COMPILING_NEWLIB -Werror=undef build.

Fix MIPS _COMPILING_NEWLIB -Werror=undef build.

I see an error

../sysdeps/mips/strcmp.S:25:7: error: "_COMPILING_NEWLIB" is not defined [-Werror=undef]
 #elif _COMPILING_NEWLIB
       ^
cc1: some warnings being treated as errors

in MIPS builds.  (This is with GCC 4.9; it's possible that the DR#412
change in GCC 5 - see
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60570> - means that
-Wundef diagnostics no longer occur for #elif conditions where a
previous group's condition was true, just as with other errors there.)
This patch duly adjusts the conditionals to test whether
_COMPILING_NEWLIB is defined.

	* sysdeps/mips/memcpy.S [_COMPILING_NEWLIB]: Change condition to
	[defined _COMPILING_NEWLIB].
	* sysdeps/mips/memset.S [_COMPILING_NEWLIB]: Likewise.
	* sysdeps/mips/strcmp.S [_COMPILING_NEWLIB]: Likewise.