gdb: add inferior::{arch, set_arch}

gdb: add inferior::{arch, set_arch}

Make the inferior's gdbarch field private, and add getters and setters.
This helped me by allowing putting breakpoints on set_arch to know when
the inferior's arch was set.  A subsequent patch in this series also
adds more things in set_arch.

Change-Id: I0005bd1ef4cd6b612af501201cec44e457998eec
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>