Revert "Revert "Disallow multiple build executions in parallel""

Bug: 36698966
Test: m -j & m -j # (run two builds at once)

This reverts commit be9cd1192b.

Change-Id: Ia15c0bf156a6de54da054eab8134f36bae0823ca

ui/build/Android.bp

@@ -30,6 +30,7 @@ bootstrap_go_package {
         "kati.go",
         "make.go",
         "ninja.go",
+        "proc_sync.go",
         "signal.go",
         "soong.go",
         "util.go",
@@ -37,6 +38,7 @@ bootstrap_go_package {
     testSrcs: [
         "environment_test.go",
         "util_test.go",
+        "proc_sync_test.go",
     ],
     darwin: {
         srcs: [