gdb: move version selection from gdb.mk to Config.in.host

This avoids duplication of the version selection between these two files.

Cc: Spenser Gilliland <spenser@gillilanding.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>