clang: make sure python3 is used for native builds
clang: make sure python3 is used for native builds

* with /usr/bin/python -> python2.7 on the host, I was seeing:
  CMake Error at work-shared/llvm-project-source-9.0.0-r0/git/lldb/cmake/modules/LLDBConfig.cmake:229 (message):
    Found incompatible Python interpreter (2.7) and Python libraries (3.7)
  Call Stack (most recent call first):
    work-shared/llvm-project-source-9.0.0-r0/git/lldb/CMakeLists.txt:20 (include)

Signed-off-by: Martin Jansa <516df3ff67e1fa5d153800b15fb204c0a1a389dc@gmail.com>
Signed-off-by: Khem Raj <729d64b6f67515e258459a5f6d20ec88b2caf8df@gmail.com>

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