[llvm-rc] [test] Fix the windres-preproc test with other mingw triple forms. NFC.

When invoking the preprocessor, llvm-windres produces a mingw
triple; if the llvm default target triple is a mingw target, it
is used as is - if not, a mingw style triple is generated.

Relax the testcase, to not strictly require "w64" as vendor (allow
the vendor field to be missing entirely), and allow both
"mingw32" and "windows-gnu" as OS/environment (where both forms are
common, while the latter is the normalized form).

Differential Revision: https://reviews.llvm.org/D134148