Replace SOFTFLOAT with __riscv_float_abi_*.

[brooks]

Summary

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.

[brooks]

I'm not in a position to test this so it will require testing before commit.

[brooks]

@br is this change sensible or should I abandon it?

[br]

In D10039#318482, @brooks wrote:

@br is this change sensible or should I abandon it?

Add fix for lib/libc/riscv/gen/fabs.S since it uses SOFTFLOAT macro

[brooks]

• Rebase.
• Remove another instance of SOFTFLOAT.