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[GOLD] Fix powerpc64 optimization of TOC accesses

Fixes a thinko.  Given code that puts variables into the TOC (a bad
idea, but some see the TOC as a small data section) this bug could
result in an attempt to optimize a sequence that should not be
optimized.

	* powerpc.cc (Target_powerpc::Scan::local): Correct dst_off
	calculation for TOC16 relocs.
	(Target_powerpc::Scan::global): Likewise.

2017-11-08  Kyle Butt  <iteratee@google.com>

        * object.cc (do_find_special_sections): Fix a thinko with memmem return
        values and check for != NULL rather than == 0.