Revert "kbuild: use -Oz instead of -Os when using clang"

The clang option -Oz enables *aggressive* optimization for size,
which doesn't necessarily result in smaller images, but can have
negative impact on performance. Switch back to the less aggressive
-Os.

This reverts commit 6748cb3c.
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -677,7 +677,7 @@ KBUILD_CFLAGS	+= $(call cc-disable-warning, format-overflow)
 KBUILD_CFLAGS	+= $(call cc-disable-warning, int-in-bool-context)
 
 ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
-KBUILD_CFLAGS	+= $(call cc-option,-Oz,-Os)
+KBUILD_CFLAGS	+= -Os
 else
 KBUILD_CFLAGS   += -O2
 endif