add floating point register saving/restoring to mips setjmp/longjmp

add floating point register saving/restoring to mips setjmp/longjmp

also fix the alignment of jmp_buf to meet the abi. linux always
emulates fpu on mips if it's not present, so enabling this code
unconditionally is "safe" but may be slow. in the long term it may be
preferable to find a way to disable it on soft float builds.