Add libcbor to the build

From https://github.com/PJK/libcbor:

libcbor is a C library for parsing and generating CBOR, the general-
purpose schema-less binary data format.

libcbor will be used by ssh to support FIDO/U2F keys. It is currently
intended only for use by ssh, and so is installed as a PRIVATELIB and is
placed in the ssh pkgbase package.

cbor_export.h and configuration.h were generated by the upstream CMake
build. We could create them with bmake rules instead (as NetBSD has
done) but this is a fine start.

This is currently disabled for the 32-bit library build as libfido2 is
not compatible with the COMPAT_32BIT hack in usb_ioctl.h, and there is
no need for libcbor without libfido2.

Reviewed by: kevans
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32347