clang: Default to compiler-rt/libcxx/libunwind when RUNTIME = "llvm"
clang: Default to compiler-rt/libcxx/libunwind when RUNTIME = "llvm"

This ensures that compiler defaults are defaulting to llvm provided
c/c++ runtimes which includes c-runtime, c++runtime, unwinder and omp
runtime, current approach is to inject commandline options but it does
not get passed in all cases and a lot of smaller/big patches are used to
make such package recipes behave

If RUNTIME = "gnu" then defaults stay to use gcc runtime as it is

Signed-off-by: Khem Raj <729d64b6f67515e258459a5f6d20ec88b2caf8df@gmail.com>

kraj/meta-clang,kraj/meta-clang,kraj/meta-clang