v0.6.0 error: rust-src component not found

[steveyko]

I have updated from v0.5.35 to v0.6.0, and 'cargo xbuild' is now throwing 'error: rust_src component not found.'

My --target is aarch64-unknown-none-softfloat.

v0.5.35 does not throw that error for me.

Here's some relevant info that might be useful.

rustup 1.22.1
rustc 1.47.0-nightly (2020-07-26)
cargo 1.47.0-nightly (2020-07-23)

[phil-opp]

Version 0.6.0 requires the new rust directory layout, which is only available since nightly 2020-07-30. See #87 for more information. I just updated the Readme in 400592f to document this requirement.