Revert "x64FPURoundMode: always set x87 precision"

This reverts commit 9de77b7c23252cbedaee993f7c56b2deb0ce2bef.
Setting x87 precision control is only supported on x86 platforms (not ARM or x64).
Merge pull request #357 from shuffle2/master

Revert "x64FPURoundMode: always set x87 precision"