cmake: compiler: Add flag template for disabling strict aliasing rule

This commit adds a template for specifying the C/C++ compiler flag for
disabling the strict aliasing rule.

It also enables this flag globally because the Zephyr codebase does not
strictly adhere to the aliasing rules specified by the C/C++ standards
and the optimisation strategies that assume these rules may end up
generating invalid code.

For instance, GCC 11 and above tend to optimise more aggressively
assuming the strict adherence to the standard aliasing rules and may
generate invalid code, when compiling Zephyr with `-fstrict-aliasing`,
that results in various run-time failures.

Note that the footprint and performance ramifications of disabling the
strict aliasing rule are negligible.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>