cmake: bintools: gnu: Update GDB executable names for Zephyr SDK 0.15

The Zephyr SDK 0.15.0 release has changed the default GDB executable to
be Python-less and introduced an alternate Python-capable variant
called `gdb-py` (note that the Zephyr SDK previously provided
`gdb-no-py` instead).

This commit updates the CMake toolchain detection routine to attempt
using the Python-capable GDB variant (gdb-py) first and, if it is not
available or fails to launch due to a missing libpython dependency,
use the Python-less GDB variant (gdb).

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>