Add a --sysroot option to build and rustc commands

[Zoxc]

When cross compiling you need to provide both a target and a sysroot (a place where all the target libraries can be found). Both cargo and rustc should provide a way to set these. rustc's --sysroot might be buggy or rust specific, see rust-lang/rust#31180 and it does not pass it on to the linker.

[alexcrichton]

This sounds like it's a use case for RUSTFLAGS rather than its own dedicated flag. All of the cross-compiling linkers I've seen in the past auto-find their own sysroot so making this a first-class option may be too front-and-center for this?

[Zoxc]

This won't be very useful if you have to pass the C sysroot in RUSTFLAGS anyway.

[carols10cents]

This sounds like it can be closed. Please reopen if I'm wrong! 🌵