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.