Don't disable all parallelism when EMCC_DEBUG is set (#13404)

This enables certain parallel phases such as running llvm-nm in parallel
when calculating system libraries to link.  Developers who what to
disable even that can continue to use `EMCC_CORES=1`.

Continue to disable parallelism in debug mode in these two cases:

1. When building system libraries/ports at link time.
2. In the test runner

While the risk using parallelism and debug mode is diminished due to
recent changes (#13329 #13370) it is still possible for emscripten
processed in debug mode to interfere with each other due to sharing
a temp directory.

See https://bugs.chromium.org/p/chromium/issues/detail?id=1171844

tests/runner.py

@@ -462,7 +462,7 @@ class RunnerCore(unittest.TestCase, metaclass=RunnerMeta):
       os.chdir(os.path.dirname(self.get_dir()))
       try_delete(self.get_dir())
 
-      if EMTEST_DETECT_TEMPFILE_LEAKS and not os.environ.get('EMCC_DEBUG'):
+      if EMTEST_DETECT_TEMPFILE_LEAKS and not DEBUG:
         temp_files_after_run = []
         for root, dirnames, filenames in os.walk(self.temp_dir):
           for dirname in dirnames:
@@ -1841,7 +1841,7 @@ def flattened_tests(loaded_tests):
 
 def suite_for_module(module, tests):
   suite_supported = module.__name__ in ('test_core', 'test_other', 'test_posixtest')
-  if not EMTEST_SAVE_DIR:
+  if not EMTEST_SAVE_DIR and not DEBUG:
     has_multiple_tests = len(tests) > 1
     has_multiple_cores = parallel_testsuite.num_cores() > 1
     if suite_supported and has_multiple_tests and has_multiple_cores:

tools/building.py

@@ -158,10 +158,6 @@ def clear():
 
 
 def get_num_cores():
-  if DEBUG:
-    # When in EMCC_DEBUG mode, only use a single core to avoid interleaving
-    # logging output and keeps things more deterministic.
-    return 1
   return int(os.environ.get('EMCC_CORES', multiprocessing.cpu_count()))
 
 

tools/system_libs.py

@@ -88,7 +88,7 @@ def run_build_commands(commands):
   # to setup the sysroot itself.
   ensure_sysroot()
   cores = min(len(commands), building.get_num_cores())
-  if cores <= 1:
+  if cores <= 1 or shared.DEBUG:
     for command in commands:
       run_one_command(command)
   else: