bsd-user/freebsd/os-syscall.c: Tracing and error boilerplate

bsd-user/freebsd/os-syscall.c: Tracing and error boilerplate

Add in the tracing and this system call not implemented boilerplate. Do
this by moving the guts of do_freebsd_syscall to freebsd_syscall. Put
the tracing in the wrapper function. Since freebsd_syscall is a
singleton static function, it will almost certainly be inlined. Fix
comments that referred to do_syscall since that was renamed some tie
ago.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>