Actually use tcmalloc and test for it

Change-Id: I61e47db48fca960b77eafaf4c8bda4b4d9517617
diff --git a/tools/bazel.rc b/tools/bazel.rc
index 568cc30..7b16af4 100644
--- a/tools/bazel.rc
+++ b/tools/bazel.rc
@@ -6,6 +6,9 @@
 # Always include debug information in the non-.stripped binaries.
 build --strip=never
 
+# Use the malloc we want.
+build --custom_malloc=//tools/cpp:malloc
+
 # Show paths to a few more than just 1 target.
 build --show_result 15
 # Dump the output of the failing test to stdout.