Performance is roughly the same, but this make it compatible with a larger number of GPUs

Luca Barbato authored Jun 03, 2019 and Luca Barbato committed Jun 10, 2019

Limited to PowerPC64 LE for now.

On older versions of glibc, clock_gettime isn't available in the main
libc, but part of a separate librt.

Only look for librt if clock_gettime isn't available otherwise.

Martin Storsjö authored Jun 06, 2019 and Janne Grunau committed Jun 07, 2019

This fixes building with raspbian compilers, that default to armv6.
The isb instruction is unavailable on armv6, and the cycle counter
register is accessed differently there as well.

This fixes issue #282.