[atomic] Allow fbl::atomic<T*>

Change-Id: I034107b21c71475615ea465de1f04cf1920ff104

[atomic] Remove memory_order_consume from fbl

Compiler support for this is nonexistent, and we are better off not
publishing this value.

Change-Id: I57444b2cd769879bf30cdca3db9847ea11cb8ef4

[fbl][atomic] Introduce fbl::atomic<bool>

Change-Id: Ic3d207094c52e4f2128ab61d67e3d5bd9521ced8

[utest][fbl] Use unused attribute on test variable definitions

Some of the tests in atomic_tests.cpp do nothing but verify that
variables can be defined of various types. The compiler rightly
reports those variables as unused, so tell it that's OK.

Change-Id: Iafe6f07a9c82e3b4d780d7e886f7eb0fd7c1e8c3

[fbl][atomic] Allow fbl::atomic of enum type

Change-Id: I57744bc763638055f9da5efc461a019e2200a514

[zircon][mxtl->fbl] Rename mxtl to fbl

Change-Id: Ie21b6498e1bfb0a7fa0315e40b9e5c3ee78646be