Don't limit ninja -j on Linux beyond ulimit -n

macOS limits the ninja j value to 1000, because ninja has a limit to the
number of open file descriptors of FD_SETSIZE, which is 1024 on Darwin.
On Linux, the ninja binary distributed on Chromium seems to be compiled
with poll.h support, so that this limitation doesn't exist:
22b778ca19/src/subprocess-posix.cc (L59)

Change-Id: I97848bb99c08fe118dbdaea525da713382373c9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4866223
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>

autoninja.py

@@ -273,12 +273,14 @@ def main(args):
             j_value = min(j_value, core_limit)
 
             # On Windows, a -j higher than 1000 doesn't improve build times.
-            # On POSIX, ninja is limited to at most FD_SETSIZE (1024) open file
+            # On macOS, ninja is limited to at most FD_SETSIZE (1024) open file
             # descriptors.
-            j_value = min(j_value, 1000)
+            if sys.platform in ['darwin', 'win32']:
+                j_value = min(j_value, 1000)
+
+            # Use a j value that reliably works with the open file descriptors
+            # limit.
             if sys.platform in ['darwin', 'linux']:
-                # Use a j value that reliably works with the open file
-                # descriptors limit.
                 j_value = min(j_value, int(fileno_limit * 0.8))
 
             args.append('%d' % j_value)