In commit e3fe6f5d 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/14091Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>