rustc: Remove __rust_crate_map_toplevel alias

It was a hack to workaround llvm bug: when
`_rust_crate_map_toplevel` is dll-exported, llvm generated
`-export:__rust_crate_map_toplevel,data` where extra underscore
is due to unnecessary mangling. Then linker tries to find
`__rust_crate_map_toplevel` symbol, but it doesn't exist.
Therefore alias was added to prevent the link failure.
(See http://llvm.org/bugs/show_bug.cgi?id=10174 for details.)

It also caused LTO issue: llvm's LTO pass removes
`__rust_crate_map_toplevel` alias for some reason I don't know,
so build failed at link time.

Fixes #12471