[libunwind] Use process_vm_readv to avoid potential segfaults

We've observed segfaults in libunwind when attempting to check for the
Linux aarch64 sigreturn frame, presumably because of bad unwind info
leading to an incorrect PC that we attempt to read from. Use
process_vm_readv to read the memory safely instead.

The s390x code path should likely follow suit, but I don't have the
hardware to be able to test that, so I didn't modify it here either.

Reviewed By: MaskRay, rprichard, #libunwind

Differential Revision: https://reviews.llvm.org/D126343