os/mac: updates for Xcode 12 on Catalina

[jonchang]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

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Forgot to do this when I updated the CI runners. Hopefully this should help contributors diagnose build issues with AppleClang 11 vs 12.

Here's what our CI says for brew config:

Homebrew Ruby: 2.6.3 => /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby
CPU: hexa-core 64-bit penryn
Clang: 12.0 build 1200
Git: 2.24.3 => /Applications/Xcode.app/Contents/Developer/usr/bin/git
Curl: 7.64.1 => /usr/bin/curl
Java: 14.0.1, 1.8.0_252
macOS: 10.15.7-x86_64
CLT: 12.0.32.2
Xcode: 12.0.1
XQuartz: 2.7.11 => /opt/X11

Apple clang version 12.0.0 (clang-1200.0.32.2)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin