Don't write to inferior_ptid in gnu-nat.c

Untested.

gdb/ChangeLog:
2020-06-18  Pedro Alves  <palves@redhat.com>

	* gnu-nat.c (gnu_nat_target::create_inferior): Switch to the added
	thread.
	(gnu_nat_target::attach): Don't write to inferior_ptid directly.
	Instead use switch_to_thread.
	(gnu_nat_target::detach): Use switch_to_no_thread
	instead of writing to inferior_ptid directly.  Used passed-in
	inferior instead of looking up the inferior by pid.