Fix builds on ARM & clarify (ARM) GPU support
Fixed a typo in architecture.h that prevented
the Neon Asimd instructions from being selected,
and updated the CPU brand detection to also look
for a new label with Tegra X1 on Ubuntu 16.04
Clarified in error messages and documentation that
Gromacs in fact does not build all supported GPU
architectures by default, explain the common cases
when things might fail, exactly what the user
should do to enable the support, and how the
support strings should be formatted.
Fixes #2287.
Change-Id: I87a2eb81ee11b78f072e3ef359a00c75eb7ec24b