backends: Add arm frame_nregs and set_initial_registers_tid.

backends: Add arm frame_nregs and set_initial_registers_tid.

This allows CFI unwinding for ARM. It relies on having .debug_frame around
which is always the case in our testsuite. All native backtrace tests PASS
on arm if debuginfo (for glibc) is installed on the system. Otherwise the
tests SKIP.

For non-debug unwinding ARM uses EXIDX tables, not .eh_frames, which
would have to be translated to CFI to do unwinding without .debug_frame
available.

Signed-off-by: Mark Wielaard <mjw@redhat.com>