[FPEnv][AArch64] Add lowering and instruction selection for STRICT_FP_ROUND

This gets selected to the appropriate fcvt instruction. Handling from there on
isn't fully correct yet, as we need to model fcvt reading and writing to fpsr
and fpcr.

Differential Revision: https://reviews.llvm.org/D73201

(cherry picked from commit 258d8dd7)