The fix for D111703 clobbered r4 both to:
- Save/restore the original lr.
- Load the address of _Unwind_Resume for LIBCXXABI_BAREMETAL.
This patch saves and restores lr without clobbering any extra
For LIBCXXABI_BAREMETAL, it is still necessary to clobber one extra
register to hold the address of _Unwind_Resume, but it seems better to
use ip/r12 (intended for linker veneers/trampolines) than r4 for this
The function also clobbers r0 for the _Unwind_Resume function's
parameter, but that is unavoidable.