`toTargetArch` in `pkgs/build-support/rust/lib/default.nix` is used to
set `CARGO_CFG_TARGET_ARCH`. This environment variable is supposed to
be the `<arch>` portion of an LLVM-style platform name:
```
<arch><sub>-<kernel>-<libc><abi>
```
Note that the pointer-width (the "64" in "x86_64" and "mips64") is
part of `<arch>`, but the endianness (the `_be` in `aarch64_be`) is
*not*.
Unfortunately at the moment nixpkgs' parsed `cpuType` has no way to
query for the three subparts (name, pointer-width, and
subarch/endianness), nor any way to ask for just the first two parts.
For now, this commit simply fixes the problem in the two cases that
matter: `mips64el` and `powerpc64le`, which I believe are the only two
platforms supported by both rust and nixpkgs which have a
"subarchitecture".
- `toRustTarget` and friends pulled out from rust tools into rust
library. Since they don't depend on any packages they can be more
widely useable.
- `build-rust-package` gets its own directory
- `fetch-cargo-tarball` gets its own directory
