Add support for Ubuntu named clang packages.

Ubuntu's clang packages have additional information appended to the end of
the version.

- Building Rust v0.5 with clang v3.0-6ubuntu3 fails.
- Building Rust v0.5 and incoming with clang v3.1-5ppa (backported from Debian)
works.

Closes #4441.
std: Stop using oldcomm
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