compiler.h: Split {READ,WRITE}_ONCE definitions out into rwonce.h

In preparation for allowing architectures to define their own
implementation of the READ_ONCE() macro, move the generic
{READ,WRITE}_ONCE() definitions out of the unwieldy 'linux/compiler.h'
file and into a new 'rwonce.h' header under 'asm-generic'.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

include/asm-generic/barrier.h

@@ -13,7 +13,7 @@
 
 #ifndef __ASSEMBLY__
 
-#include <linux/compiler.h>
+#include <asm/rwonce.h>
 
 #ifndef nop
 #define nop()	asm volatile ("nop")