gdb: remote: Save the stop reply packet from vRun and use it

gdb: remote: Save the stop reply packet from vRun and use it

The reply to the vRun packet is a stop reply, which may contain
expedited registers.  They are currently ignored and GDB will very soon
have to request them again when setting up the new inferior.

It's more efficient to save the reply and supply the expedited registers
when a new regcache is created.  This will also be useful for target
description parameters, in the case when those parameters are derived
from expedited registers.