xx-cargo: fix vendor for pkg-config and clang

[jprendes]

This should fix #107 .

[tonistiigi]

Does https://github.com/tonistiigi/xx/pull/90/files#diff-569650ba88094e023d2e66147417439ceeba349a6925868177e4628758830d3e do the same thing or are the functional differences?

This XX_VENDOR, XX_RUSTUP switching is generally a bit of a mess and would need some docs that make the behavior clear in some follow-up.

[thaJeztah]

• ^^ for discoverability; link above is to wasm rust support #90

[milas]

I ran into this today while trying to build something that uses the openssl-sys crate, which wants to use pkg-config. (Going from amd64 -> aarch64 in my case.)

Swapping to @jprendes fork worked, while #90 did not (the Rust/WASM PR branch fails with "pkg-config has not been configured to support cross-compilation" and has $TARGET = aarch64-unknown-linux-gnu).

[tonistiigi]

Discussed this with @crazy-max and he will work on a regression test for this. Without regression tests it is very hard for any change in these XX_VENDOR overrides not to break some combination of the triples that clang/rust/pkg-config expect.