libsys/arm: make vfork less of an outlier

Following the pattern of other architectures, make __sys_vfork the main
symbol and _vfork and vfork weak aliases. This avoids the need to
special case vfork in the generated list of symbols.

Differential Revision: https://reviews.freebsd.org/D44330

libsys: relocate implementations and manpages

Remove core system call implementations and documentation to lib/libsys
and lib/libsys/<arch> from lib/libc/sys and lib/libc/<arch>/<sys>.
Update paths to allow libc to find them in their new home.

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