libc: Provide sub fp(s|g)etmask() implementations for RISC-V

[kp]

Summary

RISC-V doesn't support floating-point exceptions.

RISC-V Instruction Set Manual: Volume I: User-Level ISA, 11.2
Floating-Point Control and Status Register: "As allowed by the standard,
we do not support traps on floating-point exceptions in the base ISA,
but instead require explicit checks of the flags in software. We
considered adding branches controlled directly by the contents of the
floating-point accrued exception flags, but ultimately chose to omit
these instructions to keep the ISA simple."

We still need these functions, because some applications (notably Perl)
call them, but we cannot provide a meaningful implementation.

Sponsored by: Axiado

[kp]

I've provided this as a point of discussion more than as a finished solution. I'm not sure if there's a better way to do this.

[mhorne]

FWIW Perl's usage of these functions is pretty simple, they have a single line

#define PERL_FPU_INIT (void)fpsetmask(0)

which I guess is used to ensure that floating point exceptions won't trap. Looks like we've got that covered.

Myself I can't image how to implement these as anything other than stubs. Our implementation of feholdexcept, which is similar to fpsetmask, is a stub.

[kp]

Yeah, I don't see better options either. If there are no objections I'm going to commit this in a few days.