bitbake.conf: Define TC_CXX_RUNTIME

bitbake.conf: Define TC_CXX_RUNTIME

This variable is used to denote the C/C++ compiler runtime to use.
Right now there are few alternatives out of the core which could
use this variable to define this property, the values it can take
are 'gnu' for gcc runtime. 'llvm' for using compiler-rt+libc++ and
'android' to use android runtime. Default settings is to use gnu
which is current silent default also.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>