[ASan] temporarily disable alloc_dealloc_mismatch on Mac, since the previous commit caused error reports in gTest.
Large code model support for PowerPC.

Large code model is identical to medium code model except that the
addis/addi sequence for "local" accesses is never used.  All accesses
use the addis/ld sequence.

The coding changes are straightforward; most of the patch is taken up
with creating variants of the medium model tests for large model.