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

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
Differential Revision: https://reviews.freebsd.org/D25740