cmake: Toolchain abstraction: optimizations

Introduce toolchain_cc_optimize_for_* family of macros.
Each macro represents a general optimization class.
Each macro is then responsible for setting an output variable to that
class-of-optimization's flag.
The names of these output variables are decided from the root
CMakeLists.txt.

No functional change expected.

Clang's optimization 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>