crypto/x509: use SecItemExport instead of deprecated SecKeychainItemExport

SecKeychainItemExport is deprecated as of macOS 10.7. The minimum
supported version is macOS 10.10, so use SecItemExport instead.

While at it also bump macosx-version-min to 10.10 and
__MAC_OS_X_VERSION_MAX_ALLOWED to 101300 (for macOS 10.13).

Tested on macOS 10.10, 10.11 and 10.12.

Updates #23122

Change-Id: Id4cd6a5cea93315791253dc248e40e5615760a6c
Reviewed-on: https://go-review.googlesource.com/116396Reviewed-by: 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=1080
+#cgo CFLAGS: -mmacosx-version-min=10.10 -D__MAC_OS_X_VERSION_MAX_ALLOWED=101300
 #cgo LDFLAGS: -framework CoreFoundation -framework Security
 
 #include <errno.h>
@@ -136,10 +136,7 @@ int FetchPEMRoots(CFDataRef *pemRoots, CFDataRef *untrustedPemRoots) {
 				}
 			}
 
-			// Note: SecKeychainItemExport is deprecated as of 10.7 in favor of SecItemExport.
-			// Once we support weak imports via cgo we should prefer that, and fall back to this
-			// for older systems.
-			err = SecKeychainItemExport(cert, kSecFormatX509Cert, kSecItemPemArmour, NULL, &data);
+			err = SecItemExport(cert, kSecFormatX509Cert, kSecItemPemArmour, NULL, &data);
 			if (err != noErr) {
 				continue;
 			}