use eh_frame_hdr table to speed up unwinding

use eh_frame_hdr table to speed up unwinding

* runtime/sym.h (_stp_module): add unwind_hdr_addr member.
* runtime/unwind.c (read_ptr_sect): Modification of read_pointer that
  also handles offsets from text or "data" sections.
  (read_pointer): Use it.
  (_stp_search_unwind_hdr): Use read_ptr_sect to calculate values that
  are relative to the .eh_frame_hdr section.
  (unwind_frame): Search the frame header if this is an eh frame.
* translate.cxx (get_unwind_data): Get the eh_frame_hdr too.
  (dump_unwindsyms): Write out eh_frame_hdr stuff.