clang: improve LLVM target selection logic

clang: improve LLVM target selection logic

The GPU targets are incredibly slow to build, so if the DISTRO_FEATURES
doesn't include opengl or vulkan assume that the user will not be using
a GPU and disable them.

Alternatively, a distribution could state that they'll only be using
one of the backends, and set LLVM_TARGETS_GPU explicitly.

On my build machine, disabling the GPU targets reduces the build time of
clang-native from 21m to 16m.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>