Add detection for ARMv7 cycle counter support

Add detection for ARMv7 cycle counter support

ARMv7 requires special kernel settings to allow cycle
counters to be read. This change adds a cmake setting
to enable/disable counters. On all architectures but ARMv7
it is enabled by default, and on ARMv7 we run a small test
program to see if the can be executed successfully. When
cross-compiling to ARMv7 counters will be disabled, but
either choice can be overridden by setting a value for
GMX_CYCLECOUNTERS in cmake.

Fixes #1933.

Change-Id: I1e217d7a09f84a6bcf4eb5bf4a656d430465c915