Refactor files in arch/arm64/kvm/hyp to compile all code which runs in EL2
under nVHE into separate object files from the rest of KVM. This is done in
preparation for being able to unmap .hyp.text from EL1 but has other benefits,
notably:
* safe use of KASAN/UBSAN/GCOV instrumentation on VHE code,
* cleaner HVC API,
* no need for __hyp_text annotations.
nVHE-specific code is moved to hyp/nvhe and compiled with custom build rules
similar to those used by EFI stub. Shared source files are compiled under both
VHE and nVHE build rules. Where a source file contained both VHE and nVHE code,
it is split into a shared header file and two C source files. This is done one
file per commit to make review easier.