check for std symbols

[gnzlbg]

Test whether the final rlib or binary contain any std symbols or not.

The llvm-tools-preview component contains an llvm-nm that can be used to list all symbols in a binary, and the cargo-binutils sub-command can be used to find the appropriate llvm-nm binary for the current toolchain (https://internals.rust-lang.org/t/llvm-tools-a-new-rustup-component-for-binary-inspection-objdump-nm-size-and-profiling-profdata/7830/9).

With that one can check whether the final binary contains any std library symbols, a check that must pass independently of what all other checks say.

[hobofan]

I explored that route, but ultimately got stuck on isolating the binary part out of the *.rlib files. Haven't had a look at llvm-tools-preview, though.

I'm generally pro doing that, though most likely as an optional thing if it requires llvm-tools-preview to be added as a component.