ARM: 6396/1: Add SWP/SWPB emulation for ARMv7 processors

The SWP instruction was deprecated in the ARMv6 architecture,
superseded by the LDREX/STREX family of instructions for
load-linked/store-conditional operations. The ARMv7 multiprocessing
extensions mandate that SWP/SWPB instructions are treated as undefined
from reset, with the ability to enable them through the System Control
Register SW bit.

This patch adds the alternative solution to emulate the SWP and SWPB
instructions using LDREX/STREX sequences, and log statistics to
/proc/cpu/swp_emulation. To correctly deal with copy-on-write, it also
modifies cpu_v7_set_pte_ext to change the mappings to priviliged RO when
user RO.

Signed-off-by: Leif Lindholm <leif.lindholm@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>