Makefile: Use -O3 optimization for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE

-O3 is much more stable with modern compilers these days than it was a
decade ago. Using -O3 on the kernel results in significantly improved
hackbench performance, which is a sign that overall performance in the
kernel is improved. It works especially well in conjunction with LTO.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>

Makefile

@@ -720,7 +720,7 @@ KBUILD_CFLAGS	+= $(call cc-disable-warning, attribute-alias)
 ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
 KBUILD_CFLAGS   += -Os
 else
-KBUILD_CFLAGS   += -O2
+KBUILD_CFLAGS   += -O3
 endif
 
 # Tell gcc to never replace conditional load with a non-conditional one