sframe.h: Add SFrame format definition

The header sframe.h defines the SFrame format.

The SFrame format is the Simple Frame format.  It can be used to
represent the minimal necessary unwind information required for
backtracing.  The current version supports AMD64 and AARCH64.

More details of the SFrame format are included in the documentation
of the header file in this patch.

include/ChangeLog:
	* sframe.h: New file.