use notcmalloc builder for valgrind leak checking

This reverts 453d72b294af6d3db1ae7a212ba9b43209d69f26.  It turns out that
tcmalloc defeats the leak checking in valgrind.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 279f39d4c18004ed7b005b430d3862b8b2ec2f73)