Use cargo-ndk as a RUSTC_WRAPPER to pass --target to Clang

[rib]

Okey, this is a another approach, inspired by what ndk-build does but avoiding touching CARGO_ENCODED_RUSTFLAGS, due to the huge can of worms that opens

This is an alternative workaround for #92 that's similar to the solution used in ndk-build except that the -Clink-arg= rustflag is injected via a RUSTC_WRAPPER instead of trying to modify CARGO_ENCODED_RUSTFLAGS.

It turned out to be practically impossible to be able to reliably set CARGO_ENCODED_RUSTFLAGS without risking trampling over rustflags that might be configured via Cargo (for example target.<cfg>.rustflags are especially difficult to read outside of cargo itself)

This approach avoids hitting the command line length limitations of the current workaround and avoids needing temporary hard links or copying the cargo-ndk binary to the target/ directory.

Even though we've only seen quoting issues with the Windows .cmd wrappers this change to avoid using the wrapper scripts is being applied consistently for all platforms. E.g. considering the upstream recommendation to avoid the wrapper scripts if possible:

https://android-review.googlesource.com/c/platform/ndk/+/2134712

Fixes: #92
Fixes: #104
Addresses: android/ndk#1856

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With this approach we're again able to build our project at Embark which currently fails to compile due to the command line is to long errors mentioned in #104. This solution is also compatible with the way that we configure rustflags via .cargo/config.toml.

Compared to #105 it seems better to be side stepping the Clang wrappers entirely and then not needing to patch them.

Cc: @MarijnS95

[rib]

Closing this for now in favor of #108 as a solution instead - can potentially re-open later if it makes sense.