[llvm-rc] Try to fix the Preprocessor/llvm-rc.rc test on non arm/x86 architectures

[llvm-rc] Try to fix the Preprocessor/llvm-rc.rc test on non arm/x86 architectures

When llvm-rc invokes clang for preprocessing, it uses a target
triple derived from the default target. The test verifies that
e.g. _WIN32 is defined when preprocessing.

If running clang with e.g. -target ppc64le-windows-msvc, that
particular arch/OS combination isn't hooked up, so _WIN32 doesn't
get defined in that configuration. Therefore, the preprocessing
test fails.

Instead make llvm-rc inspect the architecture of the default target.
If it's one of the known supported architectures, use it as such,
otherwise set a default one (x86_64). (Clang can run preprocessing
with an x86_64 target triple, even if the x86 backend isn't
enabled.)

Also remove superfluous llvm:: specifications on enums in llvm-rc.cpp.