Fix fork-related regressions on GNU/Linux

Fix fork-related regressions on GNU/Linux

Commit 5cd63fda035d ("Fix "Remote 'g' packet reply is too long"
problems with multiple inferiors") caused a number of regressions on
native GNU/Linux, all related to follow-fork support.  E.g.:

  src/gdb/target.c:3141: internal-error: gdbarch* default_thread_architecture(target_ops*, ptid_t): Assertion `inf != NULL' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Quit this debugging session? (y or n)
  Resyncing due to internal error.
  FAIL: gdb.base/catch-signal-fork.exp: got SIGHUP after fork (GDB internal error)

This commit fixes it.

gdb/ChangeLog:
2017-10-05  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_child_follow_fork): When following the parent
	and detaching the child, consult the parent thread's architecture
	instead of the child's.