Add atomic utilities: AtomicValue, AtomicEnumSet, and AtomicEnumFlag

Adds atomic utilities (based on raw atomic operations) for your convenience:

{AtomicValue}: A simple integer value that can be atomically
read/set/incremented.

{AtomicEnumSet}: Similar to EnumSet this set basically represents an atomic
bitfield based on enums. Atomic operations guarantee that setting one bit does
not race with setting other bits.

{AtomicEnumFlag}: A flag that is based on an enum that can be read and
(attempted to be) changed atomically.

BUG=chromium:524425
LOG=N

Committed: https://crrev.com/47d42a9ac120080fbb7d48805780d5edf18a00d8
Cr-Commit-Position: refs/heads/master@{#30560}

[Michael Lippautz, 2015-09-03 10:31:38]

mlippautz@chromium.org changed reviewers:
+ bmeurer@chromium.org, mstarzinger@chromium.org

[Michael Lippautz, 2015-09-03 10:31:39]

RFC

We definitely have the need for some atomic primitives (values/counters, sets,
flags) in heap/ in future. The idea is to encapsulate the underlying storage to
avoid using it directly. I would like to avoid re-implementing atomic counters
all the time but also prohibit unsynchronized access.

I did not add too much operator overloads to keep things simple.

wdyt in general? Also, keep it in src/, or move it to src/heap/?

[Benedikt Meurer, 2015-09-03 10:32:44]

LGTM, but please add a more descriptive CL message.

[Michael Lippautz, 2015-09-03 11:27:40]

The CQ bit was checked by mlippautz@chromium.org

[Michael Lippautz, 2015-09-03 11:27:40]

The patchset sent to the CQ was uploaded after l-g-t-m from bmeurer@chromium.org
Link to the patchset: https://codereview.chromium.org/1310993004/#ps40001
(title: "Add new file to buildsystem files")

[commit-bot: I haz the power, 2015-09-03 11:27:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1310993004/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1310993004/40001

[commit-bot: I haz the power, 2015-09-03 12:54:31]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-09-03 12:54:43]

Patchset 3 (id:??) landed as
https://crrev.com/47d42a9ac120080fbb7d48805780d5edf18a00d8
Cr-Commit-Position: refs/heads/master@{#30560}