For reasons that are unclear, the Linaro EDK2 CI is throwing errors when
building ArmCrashDumpDxe with CLANGDWARF, as the resulting build
contains non-adjacet MOVW/MOVT pairs, which cannot be relocated
correctly in PE/COFF.
Let's build it only for AARCH64 - its utility on ARM is doubtful anyway.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>