[LLDB] [PECOFF] Use a "pc" vendor name in aarch64 triples

This matches all other architectures listed in the same file.

This fixes debugging aarch64 executables with lldb-server, which
otherwise fails, with log messages like these:

Target::SetArchitecture changing architecture to aarch64 (aarch64-pc-windows-msvc)
Target::SetArchitecture Trying to select executable file architecture aarch64 (aarch64-pc-windows-msvc)

ArchSpec::SetArchitecture sets the vendor to llvm::Triple::PC
for any coff/win32 combination, and if this doesn't match the triple
set by the PECOFF module, things doesn't seem to work with when
using lldb-server.

Differential Revision: https://reviews.llvm.org/D68939

llvm-svn: 374867