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>

Kconfig.zephyr

@@ -478,6 +478,7 @@ choice COMPILER_OPTIMIZATIONS
 	prompt "Optimization level"
 	default NO_OPTIMIZATIONS    if COVERAGE
 	default DEBUG_OPTIMIZATIONS if DEBUG
+	default SIZE_OPTIMIZATIONS_AGGRESSIVE if "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "llvm"
 	default SIZE_OPTIMIZATIONS
 	help
 	  Note that these flags shall only control the compiler