Don't use gas-preprocessor with clang-cl for arm targets

Since meson 0.58.0 (released in May 2021), meson accepts adding '.S' assembly files as source files to the clang-cl compiler.

Thus, this implicitly bumps the meson requirement for clang-cl arm builds to 0.58.0, but loses the requirement of having gas-preprocessor (and a perl installation) available.