arm64: traps: Shuffle code to eliminate forward declarations

Hoist the IT state handling code earlier in traps.c, to avoid
accumulating forward declarations.

No functional change.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>