RFC: Tweak the compiler's default linkage for dylibs

When the compiler generates a dynamic library, alter the default behavior to
favor linking all dependencies statically rather than maximizing the number of
dynamic libraries. This behavior can be disabled with the existing
`-C prefer-dynamic` flag.

[Rendered](https://github.com/alexcrichton/rfcs/blob/change-prefer-dynamic/text/0000-change-prefer-dynamic.md)