Set 'rounding_mode' to 'tonearest' with '#pragma STDC FENV_ACCESS OFF'.

In strict mode the 'roundin_mode' is set to 'dynamic'. Using this pragma to
get out of strict mode doesn't have any effect on the 'rounding_mode'.
See https://godbolt.org/z/zoGTf4j1G
This patch fixes that.

Differential Revision: https://reviews.llvm.org/D147733