clang-11-bootstrap: build by bootstrap before 10.12 only

[catap]

Description

Type(s)

• [ ] bugfix
• [x] enhancement
• [ ] security fix

Tested on

macOS 10.12.6 16G2136 x86_64 Xcode 9.2 9C40b

Verification

Have you

• [x] followed our Commit Message Guidelines?
• [x] squashed and minimized your commits?
• [x] checked that there aren't other open pull requests for the same change?
• [ ] referenced existing tickets on Trac with full URL?
• [ ] checked your Portfile with port lint --nitpick?
• [ ] tried existing tests with sudo port test?
• [x] tried a full install with sudo port -vst install?
• [x] tested basic functionality of all binary files?

[catap]

@Gcenx after a while thinking I'd like to agree that we don't need to use gcc10-bootstrap when it's possible

[Gcenx]

@catap this should apply to all the new bootstrapping changes. The idea of bootstrap ports to to break dependency cycles.

[catap]

@Gcenx yep, I'm working on creating "boostrap tolchain" PG which can be used to easy manage such dependencies.

[catap]

@mascguy may I ask you to review this one?