[CMake] Make the vendor part of default mingw triples consistent

Consistently use 'w64' as vendor string; it was 'pc' for the original
i686 triple added in 91bd6c92, but the later x86_64 triple
used 'w64' as vendor, added in d6c1f37f.

When the arm triples were added in c84ad73a, the differing
vendors were copied over accidentally to the arm targets too.

When using per-target runtime directories, having inconsistent vendor
parts of the target triples is fatal.

Differential Revision: https://reviews.llvm.org/D107894