[libunwind] Add initial ARM64EC support (#138583)
ARM64EC defines `__x86_64__`, which is sufficient to make most C/C++
code behave correctly. To preserve an external ABI compatible with
x86_64, this patch uses the x86_64 context layout and implements
`unw_getcontext` by storing the appropriate aarch64 registers according
to the mapping defined by the ARM64EC ABI.
