Support gdb wrappers

Use gdbwrapper() for launching gdb.  This can be redefined to launch
one of the many gdb wrappers instead.

Example:
function gdbwrapper()
{
    ddd --debugger $ANDROID_TOOLCHAIN/$GDB -x "$@"
}

Change-Id: I3cce8a2ca1bae6d531e2388a93cb52075b21a42c