cctools: Allow multiple MacPorts clang versions to satisfy 'as', decouple from llvm variant version.

[cjones051073]

Description

cctools : Update logic to select macports clang to be used by 'as' at runtime, to be a list of possible clang versions, independent of the llvm variant cctools is built with. cctools/as will at runtime search these in order and use the first it finds. If none is found, /usr/bin/clang is then used.

gcc9/gcc10: Remove logic used to determine cctools variant used, as no longer needed given above.

Closes: https://trac.macports.org/ticket/58493

Type(s)

• enhancement

Tested on

Multiple macOS version (10.14, 10.11, 10.9).

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• 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?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?

[macportsbot]

Notifying maintainers:
@jeremyhu for port cctools.

[macportsbot]

Travis Build #6659 Errored.

Lint results

--->  Verifying Portfile for cctools
--->  0 errors and 0 warnings found.
--->  Verifying Portfile for gcc10
--->  0 errors and 0 warnings found.
--->  Verifying Portfile for gcc9
--->  0 errors and 0 warnings found.

Port cctools success on xcode10.2. Log
Port cctools success on xcode9.4. Log
Port cctools success on xcode8.3. Log
Port cctools success on xcode7.3. Log

The build timed out.

[cjones051073]

Will give this a few more days, and then unless i have comments to the contrary will merge.