[AArch64] Use MCAsmInfoMicrosoft and MCAsmInfoGNUCOFF as base classes

This matches the structure used on X86 and ARM. This requires
a little bit of duplication of the parts that are equal in both
AArch64 COFF variants though.

Before SVN r335286, these classes didn't add anything that MCAsmInfoCOFF
didn't, but now they do.

This makes AArch64 match X86 in how comdat is used for float constants
for MinGW.

Differential Revision: https://reviews.llvm.org/D49637

llvm-svn: 337755