syscall(2): make i386 less of an outlier

Unlike other architectures, i386 only defined syscall() and not
_syscall() or __sys_syscall(). The syscall() function then invoked the
desired system call directly rather than invoking syscall(2). Keep the
latter as it's marginally more efficent, but also create the
conventional _syscall() and __sys_syscall() stubs.

This avoids the need to special case syscall(2) in the symbol list
generation in libsys.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44235

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