libdwfl: Add new function dwfl_frame_reg

libdwfl: Add new function dwfl_frame_reg

Dwfl has most of the infrastructure to keep the full unwind state,
including the state of unwound registers per frame using
Dwfl_Thread_Callbacks. But there is no public API to access the state,
except for the PC (dwfl_frame_pc).

This commit adds a new function dwfl_frame_reg to get the value of the
DWARF register number in the given frame.

https://sourceware.org/bugzilla/show_bug.cgi?id=28579

Signed-off-by: Di Chen <dichen@redhat.com>