Limit number of cpus and memory

On big systems, mx dies since it spawns too many JVMs in parallel.  We now add
an option to limit the number of cpus from the command line.

Also the memory used to detect the Xlint options of javac is limited to prevent
VM crashes on parallel builds.

mx.py

@@ -101,9 +101,13 @@ def cpu_count():
     if is_jython():
         from java.lang import Runtime
         runtime = Runtime.getRuntime()
-        return runtime.availableProcessors()
+        cpus = runtime.availableProcessors()
     else:
-        return multiprocessing.cpu_count()
+        cpus = multiprocessing.cpu_count()
+    if hasattr(_opts, 'cpu_count'):
+        return cpus if cpus <= _opts.cpu_count else _opts.cpu_count
+    else:
+        return cpus
 
 try: subprocess.check_output
 except: subprocess.check_output = check_output
@@ -6463,6 +6467,7 @@ def __init__(self, parents=None):
         self.add_argument('--mx-tests', action='store_true', help='load mxtests suite (mx debugging)')
         self.add_argument('--jdk', action='store', help='JDK to use for the "java" command', metavar='<tag:compliance>')
         self.add_argument('--version-conflict-resolution', dest='version_conflict_resolution', action='store', help='resolution mechanism used when a suite is imported with different versions', default='suite', choices=['suite', 'none', 'latest', 'ignore'])
+        self.add_argument('-j', action='store', type=int, dest='cpu_count', help='the maximum number of cpus to use during build', metavar='<cpus>')
         if get_os() != 'windows':
             # Time outs are (currently) implemented with Unix specific functionality
             self.add_argument('--timeout', help='timeout (in seconds) for command', type=int, default=0, metavar='<secs>')
@@ -6479,7 +6484,6 @@ def _parse_cmd_line(self, opts, firstParse):
             parser.add_argument('--vm', action='store', dest='vm', help='the VM type to build/run')
             parser.add_argument('--vmbuild', action='store', dest='vmbuild', help='the VM build to build/run')
 
-
             # Parse the known mx global options and preserve the unknown args, command and
             # command args for the second parse.
             _, self.unknown = parser.parse_known_args(namespace=opts)
@@ -7541,7 +7545,7 @@ def getKnownJavacLints(self):
         Gets the lint warnings supported by this JDK.
         '''
         if self._knownJavacLints is None:
-            out = subprocess.check_output([self.javac, '-X'], stderr=subprocess.STDOUT)
+            out = subprocess.check_output([self.javac, '-X', '-Xms8M'], stderr=subprocess.STDOUT)
             if self.javaCompliance < JavaCompliance('1.9'):
                 lintre = re.compile(r"-Xlint:\{([a-z-]+(?:,[a-z-]+)*)\}")
                 m = lintre.search(out)