X86MCAsmInfoGNUCOFF: Set PointerSize as 8 for targeting x64. It caused DW_LNE_set_address was misemitted on x64.

FIXME: I haven't investigate whether CalleeSaveStackSlotSize should be 8.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205772 91177308-0d34-0410-b5e6-96231b3b80d8