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/+/188077Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/syscall_freebsd.go

@@ -19,12 +19,12 @@ import (
 
 const (
 	_SYS_FSTAT_FREEBSD12         = 551 // { int fstat(int fd, _Out_ struct stat *sb); }
-	_SYS_FSTATAT_FREEBSD12       = 552 // { int fstatat(int fd, _In_z_ char *path, \
-	_SYS_GETDIRENTRIES_FREEBSD12 = 554 // { ssize_t getdirentries(int fd, \
-	_SYS_STATFS_FREEBSD12        = 555 // { int statfs(_In_z_ char *path, \
-	_SYS_FSTATFS_FREEBSD12       = 556 // { int fstatfs(int fd, \
-	_SYS_GETFSSTAT_FREEBSD12     = 557 // { int getfsstat( \
-	_SYS_MKNODAT_FREEBSD12       = 559 // { int mknodat(int fd, _In_z_ char *path, \
+	_SYS_FSTATAT_FREEBSD12       = 552 // { int fstatat(int fd, _In_z_ char *path, _Out_ struct stat *buf, int flag); }
+	_SYS_GETDIRENTRIES_FREEBSD12 = 554 // { ssize_t getdirentries(int fd, _Out_writes_bytes_(count) char *buf, size_t count, _Out_ off_t *basep); }
+	_SYS_STATFS_FREEBSD12        = 555 // { int statfs(_In_z_ char *path, _Out_ struct statfs *buf); }
+	_SYS_FSTATFS_FREEBSD12       = 556 // { int fstatfs(int fd, _Out_ struct statfs *buf); }
+	_SYS_GETFSSTAT_FREEBSD12     = 557 // { int getfsstat(_Out_writes_bytes_opt_(bufsize) struct statfs *buf, long bufsize, int mode); }
+	_SYS_MKNODAT_FREEBSD12       = 559 // { int mknodat(int fd, _In_z_ char *path, mode_t mode, dev_t dev); }
 )
 
 // See https://www.freebsd.org/doc/en_US.ISO8859-1/books/porters-handbook/versions.html.