Certain installations of Xcode are affected by a bug that causes
them to print an inconsequential link-time warning that looks like:

	ld: warning: text-based stub file /System/Library/Frameworks//Security.framework/Security.tbd and library file /System/Library/Frameworks//Security.framework/Security are out of sync. Falling back to library file for linking.

This has nothing to do with Go, and we've sent this repro case
to Apple:

	$ pkgutil --pkg-info=com.apple.pkg.CLTools_Executables | grep version
	version: 10.0.0.0.1.1535735448
	$ clang --version
	Apple LLVM version 10.0.0 (clang-1000.10.44.2)
	Target: x86_64-apple-darwin17.7.0
	Thread model: posix
	InstalledDir: /Library/Developer/CommandLineTools/usr/bin
	$ cat > issue.c
	int main() { return 0; }
	^D
	$ clang issue.c -framework CoreFoundation
	ld: warning: text-based stub file /System/Library/Frameworks//CoreFoundation.framework/CoreFoundation.tbd and library file /System/Library/Frameworks//CoreFoundation.framework/CoreFoundation are out of sync. Falling back to library file for linking.
	$

Even if Apple does release a fixed Xcode, many people are seeing
this useless warning, and we might as well make it go away.

Fixes #26073.

Change-Id: Ifc17ba7da1f6b59e233c11ebdab7241cb6656324
Reviewed-on: https://go-review.googlesource.com/c/144112Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>