Implement single-/double-precision round intrinsic in x86_64

Rationale:
X86_64 does not provide a direct instruction for the
required rounding and NaN and large positive numbers
must be dealt with too. This CL generates code that
correctly implements SP and DP round.

Test: 580-checker-round

BUG=26327751

Change-Id: Ia7518e2c30afafba4e037e2d0c21e0ce926f0425