AArch64: make inexact signalling on round Darwin-specific

AArch64: make inexact signalling on round Darwin-specific

C11 leaves the choice on whether round-to-integer operations set the inexact
flag implementation-defined. Darwin does expect it to be set, but this seems to
be against the intent of the IEEE document and slower to implement anyway. So
it should be opt-in.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242446 91177308-0d34-0410-b5e6-96231b3b80d8