kconfig: llvm: Set default opt level to -Oz when building with LLVM

Experimentally, -Oz seems to give size results closer to GCC's -Os
(though GCC does now support -Oz as well). Set -Oz as the default for
LLVM to give the better codesize.

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