Fix -Wundef warnings for _ABI* on MIPS

[Roland McGrath]

I would recommend two things:

1. Give sgidefs.h clear comments that its mandate is to define only
   implementation-space macro names so that anyone considering modifying
   the file knows that must be kept name space clean so that it continues
   to be appropriate to include it in any installed header.  (While you're
   there you could clean up the existing comments to match our style.)

2. Write:
	#include <sgidefs.h>	/* Defines _MIPS_*, _ABI*.  */
   just so it's obvious what the #include is there for, since it
   has a name so far from obviously related to its purpose.