[petsc-dev] GPU timers broken in main

[Zhang, Hong]

GPU timers are currently broken in main. Event.GpuTime is always zero, so the GPU FLOPs reported in the log is zero too. Git bisect points to c708d6e3a1c9bc4418db993825b9337456e59b5c as the first bad commit. 

In this commit, the global variables in plog.c have two versions (one is thread-safe and the other is not), e.g. petsc_gtime_th and petsc_gtime. PetscLogGpuTimeBegin/End updates petsc_gtime to capture the GPU time, but petsc_gtime_th is used to update eventInfo->GpuTime for the log report. Stefano might want to take a took.

Hong