gdb: move set_target_gdbarch to inferior::set_arch

gdb: move set_target_gdbarch to inferior::set_arch

set_target_gdbarch is basically a setter for the current inferior's
arch, that notifies other parts of GDB of the architecture change.  Move
the code of set_target_gdbarch to the inferior::set_arch method.

Add gdbarch_initialized_p, so we can keep the assertion.

Change-Id: I276e28eafd4740c94bc5233c81a86c01b4a6ae90
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>