-resource-dir flag for clang should be absolute

[chancila]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

A clang based toolchain controlling compiler built-in directory via --resource-dir will not work in tandem with cgo enabled go toolchain.

This PR adds --resource-dir to the set of cgo compiler flags the builder transforms to absolute paths.

Which issues(s) does this PR fix?

Fixes #2924

Other notes for review

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[robfig]

Hi, thanks for your contribution! Is it possible to provide a test which fails without this behavior and passes now?

[chancila]

a full blown integration style test would be quite difficult because it would require creating a valid cc_toolchain, I tried to look if there's any other test in the project validating the current behavior (there isn't really) so I didn't write a test

[steeve]

@robfig giving you the final say on this of course

[robfig]

Thanks for the contribution!