unwinder: generate backtrace using SFrame format

unwinder: generate backtrace using SFrame format

[Changes in V4]
  - Renamed ESFRAME_* enum error code names to SFRAME_ERR_*.
  - Addressed review comments by Mike.
    - Use AC_CACHE_CHECK macro in sframe.m4
    - Delete config/sframe.m4. Add into libsframe/acinclude.m4.
    - Code fixups.
[End of changes in V4]

[Changes in V3]
  - Use the updated APIs from libsframe.
  - Use sframe_decoder_get_fixed_ra_offset on AMD64 instead of magic
    number -8.
[End of changes in V3]

[Changes in V2]
  - Minor formatting fixes.
[End of changes in V2]

A simple unwinder based on SFrame format.

The unwinder is made available via libsframebt library.

Buildsystem changes have been made to build libsframebt only when
--gsframe support is available in the assembler. These buildsystem
changes are necessary because the SFrame based unwinder the SFrame
unwind info for itself to work.

include/ChangeLog:

	* sframe-backtrace-api.h: New file.

ChangeLog:

	* libsframe/acinclude.m4: New file.
	* libsframe/Makefile.am: Build backtrace functionality in its
	own library.  Install libsframebt conditionally.
	* libsframe/Makefile.in: Regenerate.
	* libsframe/aclocal.m4: Regenerate.
	* libsframe/configure: Regenerate.
	* libsframe/configure.ac: Check if gas supports --gsframe
	command line option.
	* libsframe/sframe-backtrace-err.c: New file.
	* libsframe/sframe-backtrace.c: New file.