Fix mesa3d build w/C and __has_cpp_attribute

As long as we're building with C++, the compiler is supposed to ignore
unknown attributes, so we can remove the current ifdef. Also switch to
the standard [[fallthrough]] rather than the clang specific one.

Test: build mesa3d
Change-Id: Ic1545e89bf0f9585a38979fafc0562161403a103
3 files changed