INSTALL: Document programs and packages needed for using gdb_grub script

INSTALL: Document programs and packages needed for using gdb_grub script

Now that the gdb_grub script uses the Python API in GDB, a GDB with Python
support must be used. Note that this means a GDB with version greater than
7.0 must be used. This should not be an issue since that was released over
a decade ago. Also, the minimum version of Python must be 3.5, which was
released around 8 years ago.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>