[ARM] 2941/1: Fix running legacy binaries from a soft-float root filesystem with CONFIG_IWMMXT.

Patch from Daniel Jacobowitz

Thread flags are inherited on fork().  In order for a binary which has
the iWMMXt coprocessor enabled to run a binary which needs the FPA
emulation, we need to explicitly clear TIF_USING_IWMMXT if we are not
going to set it.

Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>