arm64: Initialize VFP control register.

The RW fields in this register reset to architecturally unknown values,
so initialize these to the proper rounding and denormal mode.

Approved by: re (gjb)

(cherry picked from commit 65618fdda0f272a823e6701966421bdca0efa301)
(cherry picked from commit 214ef320018db339d52e6ec9a02bf2a5691b6575)