pkg/57837: bootstrap-mk-files bsd.own.mk defines wrong OBJECT_FMT on macOS (Darwin)

[campbell+netbsd%mumble.net@localhost]

>Number:         57837
>Category:       pkg
>Synopsis:       bootstrap-mk-files bsd.own.mk defines wrong OBJECT_FMT on macOS (Darwin)
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    pkg-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Jan 12 01:05:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current
>Organization:
The NetBSDarwin Foundation
>Environment:
macOS
>Description:
On macOS (`Darwin'), bootstrap-mk-files bsd.own.mk defines OBJECT_FMT=ELF even though the correct definition is OBJECT_FMT=Mach-O.

pkgsrc's mk infrastructure has a workaround in mk/bsd.prefs.mk:

# Set this before <bsd.own.mk> does, since it doesn't know about Darwin
.if ${OPSYS} == "Darwin"
OBJECT_FMT?=            Mach-O
.endif

But it would be nice if bootstrap-mk-files defined this correctly in the first place to render this workaround unnecessary.
>How-To-Repeat:
$ cat Makefile
.include <bsd.own.mk>
$ bmake -V OBJECT_FMT
ELF
>Fix:
Yes, please!