Compiler does not gracefully handle multi-core builds

[alerque]

Re:

https://github.com/ROCm/rocFFT/blob/1680fbde578bbc65d1abfbaadf53e49dd5e0494c/library/src/rtc_compile.cpp

This is what CPU utilization looks like:

Somehow the usage is artificially capped at 50/70% of each core rather than just getting the job done.