Introduce GMX_USE_SIMD_KERNELS cmake option

Most GROMACS development does not need to recompile the SIMD nbnxm (and fep) kernels whenever their dependencies change. These dependencies are large in number, and include frequently changed files, including config.h and various utility and nbnxm module headers. This flag permits people to efficiently recompile while working on code that doesn't directly target changes to the SIMD kernels.

It also means that CI builds not aimed at efficient mdrun execution times can instead minimize compilation times and ccache db sizes. This will also have the side effect of testing more of the reference NBNXM kernels.

There's other SIMD code (particularly PME, bonded, LINCS, update) which still compiles and runs in the usual way. Currently these are less costly to compile and harder to disable. That could change in future.