When targeting windows, the .arch directive isn't available.

So far, when building for windows, we've always used gas-preprocessor,
both when using msvc's armasm and when using clang. Lately, clang/llvm
has implemented the last missing piece (altmacro support) for building
our assembly without gas-preprocessor. This means that we now build
for arm/windows with clang without any extra compatibility layer.
Signed-off-by: Martin Storsjö <martin@martin.st>