bfd: use "stack trace" instead of "unwind" for SFrame

SFrame format is meant for generating stack traces only.

bfd/
	* elf-bfd.h: Replace the use of "unwind" with "stack trace".
	* elf-sframe.c: Likewise.
	* elf64-x86-64.c: Likewise.
	* elfxx-x86.c: Likewise.

include/
	* elf/common.h: Likewise.