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

SFrame format is meant for generating stack traces only.

gas/
	* as.c: Replace the use of "unwind" with "stack trace".
	* config/tc-aarch64.c: Likewise.
	* config/tc-aarch64.h: Likewise.
	* config/tc-i386.c: Likewise.
	* config/tc-i386.h: Likewise.
	* gen-sframe.c: Likewise.
	* gen-sframe.h: Likewise.
	* testsuite/gas/cfi-sframe/cfi-sframe-aarch64-2.s: Likewise.
	* testsuite/gas/cfi-sframe/cfi-sframe-common-8.s: Likewise.
	* testsuite/gas/cfi-sframe/common-empty-2.s: Likewise.
	* testsuite/gas/cfi-sframe/common-empty-3.s: Likewise.