Fix MIPS __mips_isa_rev -Werror=undef build.

Fix MIPS __mips_isa_rev -Werror=undef build.

I see an error

In file included from ../sysdeps/mips/include/sys/asm.h:20:0,
                 from ../sysdeps/mips/start.S:39:
../sysdeps/mips/sys/asm.h:421:5: error: "__mips_isa_rev" is not defined [-Werror=undef]
 #if __mips_isa_rev < 6
     ^
cc1: some warnings being treated as errors

in MIPS builds.  As sys/asm.h is an installed header, it seems better
to test for !defined __mips_isa_rev here, instead of defining it to 0
as done in sysdeps/unix/mips/sysdep.h, to avoid perturbing any code
outside glibc that tests whether __mips_isa_rev is defined; this patch
does so.

	* sysdeps/mips/sys/asm.h [__mips_isa_rev < 6]: Change condition to
	[!defined __mips_isa_rev || __mips_isa_rev < 6].