gn build: use better triple on windows

The CMake build uses "x86_64-pc-windows-msvc". The "-msvc" suffix is
important because e.g. clang/test/lit.cfg.py matches against the
suffix "windows-msvc" to compute the presence of the "ms-sdk" and
the absence of the "LP64" feature.

Differential Revision: https://reviews.llvm.org/D68572

llvm-svn: 373899