cmake: Toolchain abstraction: C++

Introduce toolchain_cc_cpp_*-family of macros.

Move the following into the toolchain_cc_cpp_* macros:
 * Common base set of flags
 * C++ standard version
 * RTTI
 * Exceptions

These macros must be implemented by every compiler port.
These macros set the respective flags, but leaves logic and control to
the root CMakeLists.txt file.

No functional change expected.

Clang's C++ flags are compatible with gcc, and are thus
inherited.

This is motivated by the wish to abstract Zephyr's usage of toolchains,
permitting easier porting to other (commercial) toolchains.

Signed-off-by: Mark Ruvald Pedersen <mped@oticon.com>