meta-ohos-core: toolchain: Switch defaults to use clang, compiler-rt and libc++

Switch defaults to use clang, compiler-rt and libc++ -- let's test our
toolchain extensively before the release.
To go back to previous defaults, simply override the settings by putting
TOOLCHAIN = "gcc"
etc. into local.conf

Signed-off-by: Bernhard Rosenkränzer <bernhard.rosenkraenzer.ext@huawei.com>