[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!

xenia-build

@@ -8,6 +8,8 @@ Run with --help or no arguments for possible commands.
 """
 from __future__ import print_function
 from datetime import datetime
+from multiprocessing import Pool
+from functools import partial
 import argparse
 import json
 import os
@@ -1206,37 +1208,14 @@ class GenTestsCommand(Command):
             ''',
             *args, **kwargs)
 
-    def execute(self, args, pass_args, cwd):
+    def process_src_file(test_bin, ppc_as, ppc_objdump, ppc_ld, ppc_nm, src_file):
-        print('Generating test binaries...')
+        print('- %s' % src_file)
-        print('')
-
-        binutils_path = os.path.join('third_party', 'binutils-ppc-cygwin')
-        ppc_as = os.path.join(binutils_path, 'powerpc-none-elf-as')
-        ppc_ld = os.path.join(binutils_path, 'powerpc-none-elf-ld')
-        ppc_objdump = os.path.join(binutils_path, 'powerpc-none-elf-objdump')
-        ppc_nm = os.path.join(binutils_path, 'powerpc-none-elf-nm')
-
-        test_src = os.path.join('src', 'xenia', 'cpu', 'ppc', 'testing')
-        test_bin = os.path.join(test_src, 'bin')
-
-        # Ensure the test output path exists.
-        if not os.path.exists(test_bin):
-            os.mkdir(test_bin)
-
-        src_files = [os.path.join(root, name)
-                     for root, dirs, files in os.walk('src')
-                     for name in files
-                     if (name.startswith('instr_') or name.startswith('seq_'))
-                     and name.endswith(('.s'))]
 
         def make_unix_path(p):
             """Forces a unix path separator style, as required by binutils.
             """
             return p.replace(os.sep, '/')
 
-        any_errors = False
-        for src_file in src_files:
-            print('- %s' % src_file)
         src_name = os.path.splitext(os.path.basename(src_file))[0]
         obj_file = os.path.join(test_bin, src_name) + '.o'
         shell_call([
@@ -1285,6 +1264,36 @@ class GenTestsCommand(Command):
             make_unix_path(obj_file),
             ], stdout_path=os.path.join(test_bin, src_name) + '.map')
 
+    def execute(self, args, pass_args, cwd):
+        print('Generating test binaries...')
+        print('')
+
+        binutils_path = os.path.join('third_party', 'binutils-ppc-cygwin')
+        ppc_as = os.path.join(binutils_path, 'powerpc-none-elf-as')
+        ppc_ld = os.path.join(binutils_path, 'powerpc-none-elf-ld')
+        ppc_objdump = os.path.join(binutils_path, 'powerpc-none-elf-objdump')
+        ppc_nm = os.path.join(binutils_path, 'powerpc-none-elf-nm')
+
+        test_src = os.path.join('src', 'xenia', 'cpu', 'ppc', 'testing')
+        test_bin = os.path.join(test_src, 'bin')
+
+        # Ensure the test output path exists.
+        if not os.path.exists(test_bin):
+            os.mkdir(test_bin)
+
+        src_files = [os.path.join(root, name)
+                     for root, dirs, files in os.walk('src')
+                     for name in files
+                     if (name.startswith('instr_') or name.startswith('seq_'))
+                     and name.endswith(('.s'))]
+
+        any_errors = False
+
+        pool_func = partial(GenTestsCommand.process_src_file, test_bin, ppc_as, ppc_objdump, ppc_ld, ppc_nm)
+        with Pool() as pool:
+            pool.map(pool_func, src_files)
+
+
         if any_errors:
             print('ERROR: failed to build one or more tests.')
             return 1