avoid mdrun crash when rdtscp is not supported

When using rdtscp, mdrun now detects at runtime whether the CPU supports
this instruction and if this is not the case, it issues a fatal error
and instructs the user to recompile mdrun for the compute host. Note
that this will happen rarely, only when cross-compiling from a newer
host for a rather old one.

Additionally, when the user manually picks AVX, we also turn on RDTSCP
as all AVX-capable CPUs support it.

Also made CMake advanced cache option for GMX_USE_RDTSCP. This replaces
the previously hidden GMX_DISTRIBUTABLE_BUILD option.

Fixes #1428

Change-Id: I8bc884ef9ea8ea4661626b60490182ae2b302648