cmake: kconfig: Add new abstraction for -Oz optimization level

Both Clang [1] and (recently) GCC [2] support this flag to enable
additional codesize optimizations beyond -Os, possibly at the expense of
performance.

This tradeoff is worthwhile for some (and, Clang's -Oz seems to be
closer to GCC's -Os currently), so add a new abstraction for this flag
so users can select it as appropriate.

[1] https://clang.llvm.org/docs/CommandGuide/clang.html#cmdoption-O0
[2] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-Oz



Signed-off-by: Jonathon Penix <jpenix@quicinc.com>