Reland "autoninja: increase parallelism for non-HT cpu"

This reverts commit 564f1eee96.

Reason for revert:
fixed platform import

Original change's description:
> Revert "autoninja: increase parallelism for non-HT cpu"
>
> This reverts commit 06b18e4b69.
>
> Reason for revert:
> platform is not imported
>
> Original change's description:
> > autoninja: increase parallelism for non-HT cpu
> >
> > M1 mac seems to have capacity to build with higher parallelism.
> > This also remove limit in macOS.
> >
> > Bug: 1317620
> > Change-Id: I4460915c405cbb27ed977dcee631adb8753f2335
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3596361
> > Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
> > Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
> > Reviewed-by: Junji Watanabe <jwata@google.com>
> > Reviewed-by: Fumitoshi Ukai <ukai@google.com>
> > Reviewed-by: Philipp Wollermann <philwo@google.com>
> > Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
> > Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
>
> Bug: 1317620
> Change-Id: I325d0be8feca28e2f89a9191cfdb01de1f367d86
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3607513
> Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
> Owners-Override: Takuto Ikuta <tikuta@chromium.org>

Bug: 1317620
Change-Id: I2c3d817ca25cf6654864823788d3a8c473168942
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3612482
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Junji Watanabe <jwata@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>

autoninja.py

@@ -15,6 +15,7 @@ from __future__ import print_function
 
 import multiprocessing
 import os
+import platform
 import re
 import subprocess
 import sys
@@ -167,17 +168,21 @@ def main(args):
   if not j_specified and not t_specified:
     if use_goma or use_remoteexec:
       args.append('-j')
-      core_multiplier = int(os.environ.get('NINJA_CORE_MULTIPLIER', '40'))
+      default_core_multiplier = 80
+      if platform.machine() in ('x86_64', 'AMD64'):
+        # Assume simultaneous multithreading and therefore half as many cores as
+        # logical processors.
+        num_cores //= 2
+
+      core_multiplier = int(
+          os.environ.get('NINJA_CORE_MULTIPLIER', default_core_multiplier))
+
       j_value = num_cores * core_multiplier
 
       if sys.platform.startswith('win'):
         # On windows, j value higher than 1000 does not improve build
         # performance.
         j_value = min(j_value, 1000)
-      elif sys.platform == 'darwin':
-        # On Mac, j value higher than 500 causes 'Too many open files' error
-        # (crbug.com/936864).
-        j_value = min(j_value, 500)
 
       args.append('%d' % j_value)
     else: