Randomise mapped CDS address and relocate pointers by default

ADDITIONAL SYSTEM INFORMATION :
Ubuntu Focal with jdk-16+14

A DESCRIPTION OF THE PROBLEM :
The CDS archive is mapped as RWX to a static location (usually 0x800000000) and contains frequently-executed trampolines for CDS methods. This allows shellcode to be run with only an arbitrary-write vulnerability, whereas if the address was randomised a memory leak would also be required. Here's a poc using sun.misc.Unsafe to spray a 0xCC (INT3) instruction to the beginning of the CDS region, which is then executed and triggers a SIGTRAP signal:
```
import sun.misc.Unsafe;
import java.lang.reflect.Field;
class demo {
  private static Unsafe getUnsafe() throws IllegalAccessException, NoSuchFieldException {
      Field f = Unsafe.class.getDeclaredField("theUnsafe");
      f.setAccessible(true);
      return (Unsafe) f.get(null);
  }

  public static void main(String argv[]) throws Exception {
    byte opcode = (byte)(0xcc & 0xff);
    for(int i = 0; i < 500; i++) {
       getUnsafe().putByte(0x800000000l + i, opcode);
    }
  }
}
```.
Benchmarks from JDK-8231610 show around an eight-millisecond performance degradation when using a system-chosen (randomised) address and relocating pointers within the CDS archive.
=====
This could be fixed by making the default behaviour not to use FileMapHeader::_requested_base_address, and behave in the same way as if the SharedBaseAddress (when the archive was dumped) was 0.