x/sys/unix: remove the SYS___SYSCTL manual edit for darwin/amd64.

In commit e3fe6f5 I manually renamed SYS_SYSCTL to SYS___SYSCTL so that
syscall_bsd.go would continue to build, despite a rename of __sysctl to
sysctl somewhere between these two XNU versions:

    http://www.opensource.apple.com/source/xnu/xnu-2422.90.20/bsd/kern/syscalls.master
    http://www.opensource.apple.com/source/xnu/xnu-2782.1.97/bsd/kern/syscalls.master

In this commit I add a workaround that doesn't require manual edits to
autogenerated output, so regenerating the output should now cause no
diffs.

Change-Id: I0286484e7efcf9762ce39e5dcc7a8366e3798a77
Reviewed-on: https://go-review.googlesource.com/14091
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_darwin_amd64.go

@@ -73,3 +73,7 @@ func sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 }
 
 func Syscall9(num, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err syscall.Errno)
+
+// SYS___SYSCTL is used by syscall_bsd.go for all BSDs, but in modern versions
+// of darwin/amd64 the syscall is called sysctl instead of __sysctl.
+const SYS___SYSCTL = SYS_SYSCTL

unix/zsysnum_darwin_amd64.go

@@ -141,7 +141,7 @@ const (
 	SYS_LSEEK                          = 199
 	SYS_TRUNCATE                       = 200
 	SYS_FTRUNCATE                      = 201
-	SYS___SYSCTL                       = 202
+	SYS_SYSCTL                         = 202
 	SYS_MLOCK                          = 203
 	SYS_MUNLOCK                        = 204
 	SYS_UNDELETE                       = 205