Add full support for users running 64-bit clients on WOW64 applications

Xref https://groups.google.com/forum/#!topic/dynamorio-users/E2wrsADqkRc

The hard part of supporting mixed-mode (32-bit and 64-bit code in one application) has been in place for a long time. The missing pieces are around the edges:

• drrun injection: #803 (closed)
• automagically using the right flags ("-early_inject -heap_in_lower_4GB")
• properly passing anonymous options to the child (see the email link above)
• recognizing a 32-bit NtTerminateProcess as the exit (see the email link above)