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>

meta-ohos-core/conf/distro/allscenarios-linux.conf

@@ -12,6 +12,11 @@ DISTRO_VERSION = "0.1.99"
 
 TCLIBC ?= "musl"
 
+TOOLCHAIN ?= "clang"
+RUNTIME ?= "llvm"
+LIBCPLUSPLUS ?= "--stdlib=libc++"
+CLANGSDK ?= "1"
+
 INIT_MANAGER = "systemd"
 
 # Remove some of the default Poky DISTRO_FEATURES we inherited, but do not use.