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>
diff --git a/unix/zsysnum_darwin_amd64.go b/unix/zsysnum_darwin_amd64.go
index 05de76a..09de240 100644
--- a/unix/zsysnum_darwin_amd64.go
+++ b/unix/zsysnum_darwin_amd64.go
@@ -141,7 +141,7 @@
 	SYS_LSEEK                          = 199
 	SYS_TRUNCATE                       = 200
 	SYS_FTRUNCATE                      = 201
-	SYS___SYSCTL                       = 202
+	SYS_SYSCTL                         = 202
 	SYS_MLOCK                          = 203
 	SYS_MUNLOCK                        = 204
 	SYS_UNDELETE                       = 205