Can't compile core from the rust-src component

[japaric]

STR

$ rustup component add rust-src

$ cd $(rustc --print sysroot)/lib/rustlib/src/rust/src/libcore

$ cargo build
   Compiling core v0.0.0 (file:///home/japaric/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore)
error: couldn't read "../stdsimd/coresimd/mod.rs": No such file or directory (os error 2)
   --> lib.rs:199:5
    |
199 | mod coresimd;
    |     ^^^^^^^^

error: aborting due to previous error

error: Could not compile `core`.

Metadata

$ rustc -V
rustc 1.26.0-nightly (e026b59cf 2018-03-03)

Workaround

$ # switch to an older nightly
$ rustup default nightly-2018-03-03

cc @alexcrichton possibly caused by #48513; it seems that stdsimd will have to be included in rust-src

[japaric]

Also, this broke Xargo and the development workflow of all the no_std / embedded users who use it. cc @aturon