Adapt massif mmapunmap test to handle ppc64 backtrace

Adapt massif mmapunmap test to handle ppc64 backtrace

The reported backtrace on ppc64 platform reports "generic_start_main.isra.0"
in the backtrace, where other platforms typically see "main".  Adjust the
vgtest file to handle this variation.  This is similar to existing changes
as seen in deep-D.post.exp

FORGOT to add the file massif/tests/mmapunmap.post.exp-ppc64 before doing
commit 15950.

Bugzilla 368461

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15955