mips: add mips-sf subarch support (soft-float)

Userspace emulated floating-point (gcc -msoft-float) is not compatible
with the default mips abi (assumes an FPU or in kernel emulation of it).
Soft vs hard float abi should not be mixed, __mips_soft_float is checked
in musl's configure script and there is no runtime check. The -sf subarch
does not save/restore floating-point registers in setjmp/longjmp and only
provides dummy fenv implementation.