[OpenMP][DeviceRTL] Fix the issue that multiple calls to `omp_get_wtime` is...

[OpenMP][DeviceRTL] Fix the issue that multiple calls to `omp_get_wtime` is optimized out by mistake

Multiple calls to `omp_get_wtime` could be optimized out due to the function
is mistakenly marked as `readnone`. This patch fixes the issue, and also add the
support to run optimization on `libomptarget` tests.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D130179