runtime: disable memory profiler in gc_test
runtime: disable memory profiler in gc_test

This lets the test pass on PPC64 GNU/Linux, which uses a much
larger page size and thus uses more memory to hold blocks
allocated for memory profiling.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6048054
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go