SkAtomics: add asserts for valid memory orders.

Taken from http://en.cppreference.com/w/cpp/atomic/atomic and https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html, which agree.

No public API changes.
TBR=reed@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/868843003
2 files changed