Nightly broken on older libstdc++

[jrobsonchase]

As of nightly-2019-01-07, rustc fails on platforms with older libstdc++, i.e. CentOS 6.

error: couldn't load codegen backend "/home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/librustc_codegen_llvm-llvm.so": "/usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.14\' not found (required by /home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/../lib/libLLVM-8svn.so)"

$ ldd /home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/librustc_codegen_llvm-llvm.so
/home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/librustc_codegen_llvm-llvm.so: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.14' not found (required by /home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/../lib/libLLVM-8svn.so)
/home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/librustc_codegen_llvm-llvm.so: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.15' not found (required by /home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/../lib/libLLVM-8svn.so)
/home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/librustc_codegen_llvm-llvm.so: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.17' not found (required by /home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/../lib/libLLVM-8svn.so)
/home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/librustc_codegen_llvm-llvm.so: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.18' not found (required by /home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/../lib/libLLVM-8svn.so)
/home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/librustc_codegen_llvm-llvm.so: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.19' not found (required by /home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/../lib/libLLVM-8svn.so)
/home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/librustc_codegen_llvm-llvm.so: /usr/lib64/libstdc++.so.6: version `CXXABI_1.3.5' not found (required by /home/rust/.rustup/toolchains/nightly-2019-01-07-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/../lib/libLLVM-8svn.so)
...

nightly-2019-01-06 and earlier do not exhibit this behavior.

[alexcrichton]

Oops I think this was my fault! I've attempted to fix this in #57429