linux64: improve linux_support.s make rules

Previously we relied on the .s.o rule in share/mk/bsd.suffixes.mk to
tell make that linux_support.o is built from linux_support.s, even
though we do not use the .s.o rule to assemble it.

Reviewed by: sjg
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35864