[COFF, AArch64] Set the private label prefix to .L

This fixes calls to external functions starting with a capital L,
fixing errors like this:
fatal error: error in backend: assembler label 'LocalFree' can not be undefined

Differential Revision: https://reviews.llvm.org/D35079

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307317 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed