Support for jemalloc to replace dlmalloc.

To use jemalloc, add MALLOC_IMPL = jemalloc in a board config file
and you get the new version automatically.

Update the pthread_create_key tests since jemalloc uses a few keys.
Add a new test to verify memalign works as expected.

Bug: 981363

Change-Id: I16eb152b291a95bd2499e90492fc6b4bd7053836