perf, ARM: Modify kuser rmb() call to compile for Thumb-2

The Thumb-2 instruction set does not provide an encoding
for sub pc, r0, #95 as present in the rmb() definition used
by perf. This results in compilation failure when using a
compiler targetting an instruction set other than ARM.

This patch redefines rmb() for ARM by casting the address
of the kuser helper to a function pointer, therefore getting
the compiler to take care of making the call.

Patch taken against tip/master.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Jamie Iles <jamie.iles@picochip.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <1267616878-2154-1-git-send-email-will.deacon@arm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>