Clear the IT state when invoking a Thumb-2 signal handler

If a process is interrupted during an If-Then block and a signal is
invoked, the ITSTATE bits must be cleared otherwise the handler would
not run correctly.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Joseph S. Myers <joseph@codesourcery.com>
Merge pull request #2 from vicamo/bugzilla/919901/emulator-thumb2-signal-handler

Bug 910901 - B2G Emulator: A signal dispatching bug in Linux 2.6.29. r=tlee