add support for non-standard name of stdc++ library

it makes rustc compatible with gcc installation that are using
`--program-transform-name' configure flag (on OpenBSD for example).

- detects at configure the name of stdc++ library on the system

- use the detected name in llvm makefile (with enable-static-stdcpp),
  and pass it to mklldeps.py

- generate mklldeps.rs using this detected name

note that CFG_STDCPP_NAME is about stdc++ name, not about libc++. If
using libc++, the default name will be `stdc++', but it won't be used
when linking.

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