[PATCH] x86/powerpc make hardirq_ctx and softirq_ctx __read_mostly

The hardirq_ctx and softirq_ctx variables are written to on init only,

Signed-off-by: Andreas Mohr <andi@lisas.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>