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.

arch/mips/bits/fenv.h

@@ -1,3 +1,7 @@
+#ifdef __mips_soft_float
+#define FE_ALL_EXCEPT 0
+#define FE_TONEAREST  0
+#else
 #define FE_INEXACT    4
 #define FE_UNDERFLOW  8
 #define FE_OVERFLOW   16
@@ -10,6 +14,7 @@
 #define FE_TOWARDZERO 1
 #define FE_UPWARD     2
 #define FE_DOWNWARD   3
+#endif
 
 typedef unsigned short fexcept_t;