Add 32-bit ARM Neon SIMD support

This adds the low-level SIMD implementation
for 32-bit ARM Neon instructions. We will still
not generate nbnxn kernels for it; that is coming
in a future update. For this reason we will also
not enable ARM_NEON automatically in GMX_SIMD yet.
The port passes our unit tests on
tcbl04.theophys.kth.se (ARMv7).

Change-Id: I61f771970777e82fcef757ab6915e07061912957