gcc49Stdenv: re-add

In #77985 it was decided that gcc49 and gcc5 should be deprecated,
however we decided to keep gcc49[1].

I removed the commit which dropped gcc49, but apparently I staged the
gcc49Stdenv removal in a different commit. Readding this as we decided to
keep gcc49 for now.

[1] https://github.com/NixOS/nixpkgs/pull/77985#issuecomment-577411659