xtensa: add -mno-serialize-volatile to CFLAGS

By default xtensa gcc inserts memw for all volatile object accesses.
This is too pessimistic for the kernel: there should be no "normal"
volatile objects, and all special objects, like MMIO or objects shared
between CPUs should have explicit barriers.
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>

arch/xtensa/Makefile

@@ -46,6 +46,7 @@ KBUILD_CFLAGS += -ffreestanding -D__linux__
 KBUILD_CFLAGS += -pipe -mlongcalls
 
 KBUILD_CFLAGS += $(call cc-option,-mforce-no-pic,)
+KBUILD_CFLAGS += $(call cc-option,-mno-serialize-volatile,)
 
 ifneq ($(CONFIG_LD_NO_RELAX),)
 LDFLAGS := --no-relax