cmd/link: sort HOSTOBJ and UNDEFEXT symbols with undefined symbols in Mach-O symbol...

cmd/link: sort HOSTOBJ and UNDEFEXT symbols with undefined symbols in Mach-O symbol table

HOSTOBJ and UNDEFEXT symbols are actually not defined in the
current translation unit. So put them as undefined symbols, along
with DYNIMPORT.

For #61229.

Change-Id: I1112ca928cdc401bac45adaa3f88b19422813960
Reviewed-on: https://go-review.googlesource.com/c/go/+/503539
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>