Makefile: Fix device not booting with GCC 7.x and above

@nathanchance: Filtered from the original commit (zanezam/boeffla-kernel-cm-n51x0@228cc26)

I tested all of the flags in the above commit and this is the only
one that prevented the phone from booting, with -Os, -O2, and -O3.

Without this, the phone will boot to the Google logo, hang for a
few seconds, then reboot, and repeat the process.

This flag can be read about below. This issue can also be resolved
(supposedly) with the -mstrict-align flag, leading me to believe it
is something with memory access. I believe this is a better solution
though given that ARM handles unaligned access perfectly fine.

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-fstore-merging
https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html#index-mstrict-align

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

Makefile

@@ -587,6 +587,9 @@ KBUILD_CFLAGS	+= $(call cc-disable-warning,unused-const-variable,)
 # Disable format-truncation warnings
 KBUILD_CFLAGS   += $(call cc-disable-warning,format-truncation,)
 
+# Needed to unbreak GCC 7.x and above
+KBUILD_CFLAGS   += $(call cc-option,-fno-store-merging,)
+
 include $(srctree)/arch/$(SRCARCH)/Makefile
 
 ifdef CONFIG_READABLE_ASM