libc: make syscall stubs empty for shared lib

They are always replaced by libsys so just make them empty. In
https://reviews.freebsd.org/D14609 x86 variants call abort2, but that
requires per-arch assembly and should be of low value in the steady
state.

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