simd: revamp the (templated) decr3Hsimd() subroutine

simd: revamp the (templated) decr3Hsimd() subroutine

decr3Hsimd<stride>(m, a0, a1, a2) replaces three decrHsimd():

- decrHsimd(m, a0);
- decrHsimd(m + stride, a1);
- decrHsimd(m + 2*stride, a2);

providing a given architecture the opportunity to perform additional optimizations.

A simple wrapper is added to each architecture where
GMX_SIMD_HAVE_HSIMD_UTIL_{FLOAT,DOUBLE} is 1.

Refs !567