crypto/x509: update __MAC_OS_X_VERSION_MAX_ALLOWED on Mac

Reportedly, -mmacosx-version-min=10.6 -D__MAC_OS_X_VERSION_MAX_ALLOWED=1060
is problematic.

It means min 10.6 and max 10.6, thus exactly 10.6. But we only support
10.8+.

It never caused us problems, because we build on Macs, but apparently
if you cross-compile from Linux with some Mac compiler SDK thing, then
things break?

This was added in https://golang.org/cl/5700083 for #3131, and the
intent at the time was to pin to exactly 10.6. So it wasn't a mistake,
but it is definitely outdated.

Given that we now support 10.8 as the min, update it to 1080.

Fixes #17732

Change-Id: I6cc8ab6ac62b8638a5025952b830f23e8822b2a6
Reviewed-on: https://go-review.googlesource.com/32580
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/root_cgo_darwin.go

@@ -7,7 +7,7 @@
 package x509
 
 /*
-#cgo CFLAGS: -mmacosx-version-min=10.6 -D__MAC_OS_X_VERSION_MAX_ALLOWED=1060
+#cgo CFLAGS: -mmacosx-version-min=10.6 -D__MAC_OS_X_VERSION_MAX_ALLOWED=1080
 #cgo LDFLAGS: -framework CoreFoundation -framework Security
 
 #include <errno.h>