Allow system processes to use vfork

java.lang.ProcessBuilder is internally using vfork in the current
configuration (see UNIXProcess_md.c in libcore).

Test: On an x86 Chromebook, trigger dropbox error event (1. settings
      put global logcat_for_system_app_anr 50; 2. kill -19 `pidof
      com.android.settings'; touch Settings).  Saw seccomp error in
      logcat but only without this fix.
Test: fyi, not reproducible on walleye (arm64)
Bug: 74441404

Change-Id: I1f40ae0fe0607c5834ecbe040ed31c4c0e42131d