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: Max Filippov <jcmvbkbc@gmail.com>