libsys/riscv: Remove pointless MD syscall(2)

This file is functionally identical to the stub generated by
Makefile.sys once the MD version is removed.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44232

libc: Move per-arch sys/Makefile.inc to libsys

libc/<arch>/sys/Makefile.inc -> libsys/<arch>/Makefile.sys.

Require that libsys/<arch>/Makefile.sys exist. At least for current
archtiectures, it's not possible for an architecture to not have and MD
syscall bits.

powerpcspe/Makefile.sys's structure means it had to be modified when moved
so rename detection won't work, but it has trivial contents so the
history is unimportant.

Reviewed by: kib, emaste, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/908