libdwarf: Use the cached strtab pointer when reading string attributes.

libdwarf: Use the cached strtab pointer when reading string attributes.

Previously we would perform a linear search of the DWARF section
list for ".debug_str".  However, libdwarf always caches a pointer to
the strtab image in its debug descriptor.  Using it gives a modest
performance improvement when iterating over the attributes of each
DIE.

Reviewed by:	emaste
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D20759