[Build] Add parallel PPC test generation

Utilizes `multiprocessing` to allow for multiple power-pc assembly tests
to be generated in parallel.

Some results on my i9-11900k(8c/16t):

Before:
```
Measure-Command {.\xb gentests}

Days              : 0
Hours             : 0
Minutes           : 0
Seconds           : 11
Milliseconds      : 200
Ticks             : 112007585
TotalDays         : 0.000129638408564815
TotalHours        : 0.00311132180555556
TotalMinutes      : 0.186679308333333
TotalSeconds      : 11.2007585
TotalMilliseconds : 11200.7585
```

After:
```
Measure-Command {.\xb gentests}

Days              : 0
Hours             : 0
Minutes           : 0
Seconds           : 5
Milliseconds      : 426
Ticks             : 54265895
TotalDays         : 6.28077488425926E-05
TotalHours        : 0.00150738597222222
TotalMinutes      : 0.0904431583333333
TotalSeconds      : 5.4265895
TotalMilliseconds : 5426.5895
```

This is an over **x2** speedup!