removed x86 specifics from nbnxn SIMD kernels

removed x86 specifics from nbnxn SIMD kernels

All x86 specific preprossing and SIMD intrinsics in
nbxn_kernel_simd_???_outer/inner.h have been replaced by generic SIMD
macro code. Exclusion masks are now loaded iso set and the treatment
is the same for most setups and can use only floating point intrinsics.
Moved half-width SIMD from gmx_simd_macros.h to two 2xnn .h files.
Half-width load macros now take the output as an argument,
in preparation for Intel MIC SIMD instrinsics.
Replaced _SSE suffixes by _S in the abovementioned files.

Change-Id: Ib634726af6e51a99c904e05528b0ac10ecbc2462