`spirv-remap` is not present in modern Vulkan SDK versions, it was replaced
with the `--canonicalize-ids` pass in `spirv-opt`.

Overall, canonicalization provides a significant compression improvement,
which is important considering that currently Xenia is distributed in a ZIP
archive and contains many very similar shaders.

With normal DEFLATE compression, canonicalization reduced the size of a ZIP
with `xenia.exe` from 3.54 MB to 3.45 MB in a test done before committing.

Also disable stripping of debug information from shaders, which apparently
was among what `spirv-remap` was doing with `--do-everything`, as binding
and uniform buffer member names heavily aid in debugging in RenderDoc.

Partially integrated from #2329.

Co-authored-by: Herman S. <429230+has207@users.noreply.github.com>
Co-authored-by: Gliniak <Gliniak93@gmail.com>

Use the _xe suffix instead of the xesl_ prefix for quicker visual
recognition of identifiers, also switch to snake_case for consistency.

Also add the f suffix to float32 literals because the Metal Shading
Language is based on C++.

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!

Use variable for Python version to make upgrading easier.
xb.bat:
 Update copyright date.
 Add candidate paths.
xb.ps1
 Properly use found python executable.
 More consistency with .bat.
 Don't spew unnecessary errors, etc.
 EOF newline.