Add support for host32 for DIRDEPS_BUILD

Allow building 32bit libs for host.

Move CFLAGS additions from local.sys.dirdeps.mk (which is too early
and impacts CFLAGS defaults) to local.sys.mk

Reviewed by: stevek
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D41946

Add sys.dirdeps.mk to share/mk FILES

A few recent makefiles should have been added to FILES.
Rename sys.machine.mk to local.sys.machine.mk as it is very
tree specific so does not belong in /usr/share/mk/

Reviewed by: stevek
Differential Revision: https://reviews.freebsd.org/D41642