tcmode-default.inc: set LLVMVERSION to a major version wildcard

There is no longer need to be specific: none of the recipes
using this variable in core need to know the particular version
of llvm exactly when using the default one set via PREFERRED_VERSION
from LLVMVERSION.

This as well enables automated version updates of llvm
for minor releases.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>