tcmalloc: Use C++11 atomics where appropriate.

Reland now that we have a newer sysroot.

Ports these CLs to tcmalloc:
https://codereview.chromium.org/636783002/
https://codereview.chromium.org/1466833002/ (except mac)

No intended behavior change, but it should remove
the static initializer in atomicops_internals_x86_gcc.h
on Linux.  It's also less code.

BUG=94925,559247,572525

Committed: https://crrev.com/e13537fe418eff11d3cab9077f6a647d7c74f103
Cr-Original-Commit-Position: refs/heads/master@{#366904}
Review-Url: https://codereview.chromium.org/1549913002
Cr-Commit-Position: refs/heads/master@{#464440}
9 files changed