Update to nightly-2020-12-11.

[eddyb]

This is specifically for rust-lang/rust#79801 - I'm not sure what else got fixed, but the added test used to fail with this:

error: OpBitcast on ptr without AddressingModel != Logical
    --> /home/eddy/.cargo/registry/src/github.202132.xyz-1ecc6299db9ec823/num-traits-0.2.14/src/float.rs:1941:30
     |
1941 |     let bits: u32 = unsafe { mem::transmute(f) };
     |                              ^^^^^^^^^^^^^^^^^
     |
     = note: Stack:
             num_traits::float::integer_decode_f32
             <f32 as num_traits::float::FloatCore>::integer_decode
             <f32 as num_traits::float::FloatCore>::is_sign_negative
             <f32 as num_traits::float::FloatCore>::signum
             <f32 as num_traits::float::Float>::signum
             test_project::main
             Unnamed function ID %51