Read stop_pc after updating the gdbarch when exec'ing

When an inferior execs and changes architecture (e.g. 64 bits to 32
bits), the gdbarch associated to the inferior is updated by the
follow_exec call in handle_inferior_event_1.  We should avoid doing any
register read before that point, because the registers sent by the
remote side will be those of the new architecture, but we would
interpret them using the old architecture.  We do just that by setting
stop_pc during this window, which obviously requires reading the
registers.  This results in gdb.multi/multi-arch-exec.exp failing, GDB
outputting the following error:

  Truncated register 50 in remote 'g' packet

This patch fixes that by postponing the setting of stop_pc to after
we've updated the inferior gdbarch.

This bug was hiding another problem, and as such introduces some
failures in gdb.base/foll-exec-mode.exp.  The following patch takes care
of that.

gdb/ChangeLog:

	* infrun.c (handle_inferior_event_1): When exec'ing, read
	stop_pc after follow_exec.