Implement __flt_rounds() for RISC-V. RISC-V is "interesting" since it

Implement __flt_rounds() for RISC-V.  RISC-V is "interesting" since it
implements a variation on the traditional "to nearest" rounding mode that
rounds away from zero when tied.  The upcoming C2x includes support for that
and LLVM already implements this so provide an implementation that matches
our system compiler.

ok drahn@