There's a merged pull request on the void-packages repository

rust: update to 1.38.0, cargo: update to 0.39.0
https://github.com/void-linux/void-packages/pull/15178

Description:
Updated ppc distfiles already at https://repo.voidlinux-ppc.org/distfiles/ and ~~pending @Gottox merge~~mirrored into `alpha.de.repo.voidlinux.org`.

This also sets rust as nocross for now, to give native builders time to build first.

The 32-bit PowerPC (glibc) distfiles are custom for this release (and at least until the fixes have made it upstream) as there are ABI violation bugs that we are currently patching, which also break bootstrap (a patched compiler is necessary to build the compiler with the patches).

We're sticking with LLVM8 for now, as my testing with LLVM9 has revealed that some targets (at very least 32-bit ppc) generate broken executables with LLVM9. This cannot be conditionalized as Rust is an everything-crosscompiler. We can switch to LLVM9 once it's confirmed working (I raised it with the relevant people) - the bootstrap snapshots are made with static llvm so a switch should be fairly seamless with no bootstrap rebuilds.

This also updates cargo to 0.39 and deals with the libgit2 API/ABI breakage, in a kinda ugly (well, it is Rust...) but for now satisfactory way.

As far as I'm concerned, this should be ready to go, but testing pending for at least:

- [ ] x86_64
- [ ] x86_64-musl
- [ ] aarch64
- [ ] armv7l

Hopefully the current llvm8/9 merge fallout will be done by that time... :P

@jnbr