iOS: Enable non-pixman NEON drawhelpers for both 32 and 64-bit

The pixman drawhelpers are implemented using GAS syntax, which the
Clang assembler doesn't handle, nor do they work on 64-bit ARM, so
we disable them selectively. They are only used for 16-bit surfaces
anyways, so it not a big deal on iOS.

(cherry picked from commit 49568df9

)
Change-Id: I70848981c794739d40a6a2d9a537307bc80d61a6
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>