When building hwasan on aarch64, the DT_AARCH64_BTI_PLT flag is missing
from libclang_rt.hwasan.so because some object files without
DT_AARCH64_BTI_PLT are linked in the final DSO.
These files are specific to riscv64 and x86_64, ending up with no
aarch64 code in them.
Avoid building and linking architecture-specific files unless the
architecture is listed in HWASAN_SUPPORTED_ARCH.
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