clang: improve reproducibility

[raburaibu]

Currently, class-target is reproducible, but class-nativesdk is not. What I did:

• noticed that BuildVariables.inc contains meaningless path fragments after running sed, so I adjusted sed commands to remove the rest of paths as well;
• moved common code into a function, which is then called twice;
• changed do_compile:prepend into do_configure:append, this is more conventional and intuitive;
• verified that the resulting code works for both target and nativesdk after these changes.

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[kraj]

thanks for doing this LGTM