Switch __ARM_ARCH_7S__ to __ARM_ARCH_EXT_IDIV__ for use of sdiv/udiv assembly.

__ARM_ARCH_EXT_IDIV__ is the define that ARM is using to indicate the presence
of hardware integer divide (sdiv/udiv). Previously, this code was only being
invoked for processors marked 7S. We now can correctly generate hardware
divides on cortex-a15 devices.