benchmark

Always use inline asm DoNotOptimize with clang. (#452)

* Always use inline asm DoNotOptimize with clang.

clang-cl masquerades as MSVC but not GCC, so it was using the
MSVC-compatible definitions of DoNotOptimize and ClobberMemory.
Presumably, it's better in general to use the targeted assembly for
this functionality (the codegen is different), but the specific issue
is that clang-cl deprecates the usage of _ReadWriteBarrier, and this
gets rid of that warning.

* triggering another AppVeyor run