MIPS: make mipsisa32 and mipsisa64 link more systematic

MIPS: make mipsisa32 and mipsisa64 link more systematic

Introduce `static const struct mips_mach_extension mips_mach_32_64[]`
and `mips_mach_extends_32_64 (unsigned long base, unsigned long extension)`,
to make mipsisa32 and mipsisa64 interlink more systemtic.

Normally, the ISA mipsisa64rN has two subset: mipsisa64r(N-1) and
mipsisa32rN. `mips_mach_extensions` can hold only mipsisa64r(N-1),
so we need to introduce a new instruction `mips_mach_32_64`, which holds the pair 32vs64.

Note: R6 is not compatible with pre-R6.

bfd/ChangeLog:

	* elfxx-mips.c (mips_mach_extends_p): make mipsisa32 and
	  mipsisa64 interlink more systematic.
	  (mips_mach_32_64): new struct added.
	  (mips_mach_extends_32_64): new function added.