llvm: add __fallthrough macro for Clang

LLVM defines __GNUC__ to 4 so the __fallthrough is nothing
through include/toolchain/gcc.h. So add a few lines for
a functional __fallthrough macro for LLVM. The fallthrough
attribute is supported since Clang 10.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
1 file changed