[AArch64] Remove check for Darwin that was needed to decide if x18 should

[AArch64] Remove check for Darwin that was needed to decide if x18 should
be reserved.

The decision to reserve x18 is going to be made solely by the front-end,
so it isn't necessary to check if the OS is Darwin in the backend.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243308 91177308-0d34-0410-b5e6-96231b3b80d8