Replace SOFTFLOAT with __mips_soft_float, which gcc/clang define for us.

D8376 extended softfloat/hardfloat support, but used a macro that never
actually gets set except in libc and msun's Makefile.inc. So libc and libm
got built correctly, but any program including fenv.h itself assumed it was
on a hardfloat systen and emitted inline fpu instructions for
fedisableexcept() and friends.

Using __mips_soft_float makes everything work in all cases, since it's a
compiler-internal macro that is always set correctly for the target

PR: 217845
Submitted by: Dan Nelson <dnelson_1901@yahoo.com>
MFC after: 1 week