Remove implicit fallthrough from AArch64 to Mips in lib selection

Nothing useful to AArch64 will (should!) be found in any Mips-specific
directories.

Patch by Luke Zarko.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183956 91177308-0d34-0410-b5e6-96231b3b80d8