[7.6] Add additional helper to find prebuilt Python path.

The bulk of the change is picked up from AOSP gdb 7.3.x.
New support is added to find the prebuilt Python for Android platform
debugging.

Change-Id: I0348ab0397ac84bcfc8d82b83e2f12113c599304

Author: Ben Cheng <bccheng@google.com>
Date:   Fri Jun 21 17:31:51 2013 -0700