Fix r212522 - [mips] Improve encapsulation of the .MIPS.abiflags implementation and limit scope of related enums

Added two lines that should have been in r212522.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212523 91177308-0d34-0410-b5e6-96231b3b80d8