target-arm/helper.c: Don't flush the TLB if SCTLR is rewritten unchanged

target-arm/helper.c: Don't flush the TLB if SCTLR is rewritten unchanged

Linux makes a habit of writing the same value to the SCTLR that it
already holds. In a sample boot of the kernel to a shell prompt
it wrote the SCTLR with the value it already held 325465 times,
and wrote different values just 3 times.

Skip flushing the TLB if the SCTLR value isn't actually being changed;
this speeds up my sample boot by 3-5%.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1399560029-19007-1-git-send-email-peter.maydell@linaro.org