-Z gen-crate-map should be a crate attribute

[alexcrichton]

Crates should be able to opt-in to requesting a crate map.

An error should be generated if this attribute is specified when an rlib is being created

[alexcrichton]

Note that I think this is a suitable replacement for #11080 because you have to explicitly opt-in to it.

[larsbergstrom]

We need this in Servo because we generate a top-level shared library instead of an executable on Android so that we can create an executable using the Android NDK that has all the proper googly bits. But, we'd still like to be able to set RUST_LOG.