The #[global_allocator] used by relibc is only relibc's business.
Symbols should therefore not leak if another applications for example
wants to override the system allocator (and for bootstrap, which links
to relibc but uses its own #[global_allocator] based on libc functions).

Rust's libstd for example uses environ when using the `envs` builder
method for spawning processes, and therefore relibc cannot simply assume
environ will always point to the internal relibc env var Vec.

Update dependencies for rust 2022-03-18.

See merge request !342