Reduce libstand Makefile duplication

Userboot's copy of the libstand Makefile had more extensive changes
compared to the one in sys/boot/libstand32, but it turns out these are
not intentional and we can just include lib/libstand/Makefile as done
for libstand32 in r293040.

Reviewed by: imp, jhb
Tested by: allanjude
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D4793