xtensa: enable KCSAN

Prefix arch-specific barrier macros with '__' to make use of instrumented
generic macros.
Prefix arch-specific bitops with 'arch_' to make use of instrumented
generic functions.
Provide stubs for 64-bit atomics when building with KCSAN.
Disable KCSAN instrumentation in arch/xtensa/boot.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Marco Elver <elver@google.com>

arch/xtensa/lib/Makefile

@@ -8,3 +8,5 @@ lib-y	+= memcopy.o memset.o checksum.o \
 	   divsi3.o udivsi3.o modsi3.o umodsi3.o mulsi3.o \
 	   usercopy.o strncpy_user.o strnlen_user.o
 lib-$(CONFIG_PCI) += pci-auto.o
+lib-$(CONFIG_KCSAN) += kcsan-stubs.o
+KCSAN_SANITIZE_kcsan-stubs.o := n