Save and restore floating point registers via 64-bit
load/stores when possible. Use assembler's builtin macro
ops to generate pairs of 32-bit load/stores on Mips I cpus.
Some cpus or FR modes have only 16 even-numbered dp fp regs.
This is exposed by _MIPS_FPSET, defined by existing compilers.
Change-Id: I7f617a3ffea8da41c402ef3a68ab32c91d3d7622