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