Replace SOFTFLOAT with __riscv_float_abi_*.

With SOFTFLOAT, libc and libm were 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.

Unlike r315424 which did this for MIPS, I've used riscv_float_abi_soft
and riscv_float_abi_double macros as appropriate rather than using
__riscv_float_abi_soft exclusively. This ensures that attempts to use an
unsupported hardfloat ABI will fail.

Reviewed by: br
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10039