Re-work register size macros for MIPS.

gas/
	* config/tc-mips.c (mips_set_options): Rename gp32 to gp throughout.
	(HAVE_32BIT_GPRS, HAVE_64BIT_GPRS): Remove. Re-implement via GPR_SIZE.
	(HAVE_32BIT_FPRS, HAVE_64BIT_FPRS): Remove. Re-implement via FPR_SIZE.
	(GPR_SIZE, FPR_SIZE): New macros. Use throughout.
S390: Fix erroneous offset in fill_gregset.

This fixes a bug that leads to various failures when debugging a
31-bit inferior with a 64-bit gdb on s390x.