headers: optionalise some generated resource headers

This splits out the generated headers and conditonalises them upon the
target being enabled.

The motivation here is that the RISCV header alone added 10MB to the
resource directory, which was previously at 10MB, increasing the build
size and time. This header is contributing ~50% of the size of the
resource headers (~10MB).

The ARM generated headers are contributing about ~10% or 1MB.

This could be extended further adding only the static resource headers
for the targets that the LLVM build supports.

The changes to the tests for ARM mirror what the RISCV target already
did and rnk identified as a possible issue.

Testing:
  cmake -G Ninja -D LLVM_TARGETS_TO_BUILD=X86 -D LLVM_ENABLE_PROJECTS="clang;lld" ../clang
  ninja check-clang

Differential Revision: https://reviews.llvm.org/D112890
Reviewed By: craig.topper

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