llvm::sys::getHostCPUFeatures (lib/Support/Host.cpp): https://llvm.org/doxygen/Host_8cpp_source.html#l01427
only supports Linux's /proc/cpuinfo on non-x86 architectures.
One known user of that API is lang/ponyc, where I contributed a workaround: https://github.com/ponylang/ponyc/commit/81ba52c90cb87a049be8052754a8b031de59593d
There should be versions of this function for FreeBSD/aarch64 (by reading ID_AA64ISAR0_EL1/ID_AA64PFR0_EL1/ID_AA64ISAR1_EL1 in userspace) and armv\d|powerpc(64)? (via elf_aux_info).
If we implement anything for this, it should be reviewed upstream first. That said, I know next to nothing about ARM CPU features... :)