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

[mstorsjo]

Summary

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

[mgrang]

LGTM.

lib/Target/AArch64/MCTargetDesc/AArch64MCAsmInfo.cpp

AArch64MCAsmInfoELF::AArch64MCAsmInfoELF(const Triple &T) {
 
   UseIntegratedAssembler = true;
 
   HasIdentDirective = true;
 }
 
 AArch64MCAsmInfoCOFF::AArch64MCAsmInfoCOFF() {
   CommentString = ";";
+  PrivateGlobalPrefix = ".L";
+  PrivateLabelPrefix = ".L";
 }