syscall: include complete prototype of system calls

The prototypes were truncated because they were on multiple lines
of the source file.

Copied from the STD lines for these functions in
https://svn.freebsd.org/base/stable/12/sys/kern/syscalls.master

Change-Id: I618a5444f1353aabee2758c06f86ad726185d19c
Reviewed-on: https://go-review.googlesource.com/c/go/+/188077


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>