ARM: 7442/1: Revert "remove unused restart trampoline"

This reverts commit fa18484d.

We need the restart trampoline back so that we can revert a related
problematic patch 6b5c8045

 ("arm: new
way of handling ERESTART_RESTARTBLOCK").

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>