Fix AArch64 global-merge backend option name.

-mglobal-merge is translated to the appropriate backend option in
the driver. r277322 changed the AArch64 option name in the backend,
but the driver was never updated.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@292192 91177308-0d34-0410-b5e6-96231b3b80d8