Factor out the common code for function boundary tracing instead of
duplicating the entire implementation for both x86 and powerpc. This makes
it easier to add support for other architectures and has no functional
impact.

Phabric:	D613
Reviewed by:	gnn, jhibbits, rpaulo
Tested by:	jhibbits (powerpc)
MFC after:	2 weeks
* Use standard net/sff8472.h header for sff bits and offsets.
* Convert sff_8472_id to 'const char *' to please clang.

Pointed by:	np