[llvm-rc] Don't rewrite the arch in the default triple unless necessary

When the default target arch isn't one that is supported as a
windows target, we want to set a suitable architecture (so that
Clang tests that run plain 'llvm-rc' succeed checks for e.g.
"#ifdef _WIN32" even for llvm builds that default to e.g. ppc64).

But if the default target architecture is usable, don't rewrite it.
(Rewriting it, by e.g. "T.setArch(T.getArch())", normalizes the
spelling of the architecture, e.g. changing i686 to i386. Such a
change can make clang unable to find the right sysroot.)

This can't, unfortunately, practically be tested very well because
it is entirely dependent on the default triple of the llvm build.

Differential Revision: https://reviews.llvm.org/D104589