Manually manage thread pool stacks.

We now allocate the thread pool worker stack using a MemMap. This
enables us to name the maps so that we get more descriptive output
for debugging leaks.

Appears to fix the mips build 5/5 successful clean-oat and builds.
This is probably since glibc caches up to 40 MB of thread stacks
before releasing them.

Change-Id: I1df2de50cb95838aa0d272a09807021404ba410c
6 files changed