Assemble .s to .o using cc, not as

As of commit fd71da37d478 we no longer have an as in the default
toolchain. Although we do not make use of this rule in the base system
some ports or downstream projects might. Use cc -x assembler instead
of as.

Reviewed by: arichardson
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35859