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.
(cherry picked from commit dd37251c473e1483faba0fd5aaf30e7a55582e8a)
Change-Id: Ibd43653701a363a77af85121d3cbd229d132a06a