Add FPU support for MIPS setjmp(3)/longjmp(3).

This change saves/restores the callee-saved MIPS floating point
registers as documented by the o32/n32/n64 spec ("MIPSpro N32
ABI Handbook", Table 2-1) for the _setjmp(3), _longjmp(3),
setjmp(3) and longjmp(3) C library functions. This is only
included when the C library is built with hardware floating point
support (or when "SOFTFLOAT" is not defined).

Submitted by: sson
MFC after: 1 month
Sponsored by: DARPA, AFRL