[rustbuild] x.py build --stage 1 src/rustc --host=$HOST errors

[japaric]

with:

(In this example: BUILD = x86_64-unknown-linux-gnu and HOST = arm-unknown-linux-gnueabi)

Building stage0 std artifacts (x86_64-unknown-linux-gnu -> arm-unknown-linux-gnueabi)
(..)
Building stage0 test artifacts (x86_64-unknown-linux-gnu -> arm-unknown-linux-gnueabi)
(..)
Building LLVM for arm-unknown-linux-gnueabi
(..)
ninja: error: '/home/japaric/build/x86_64-unknown-linux-gnu/llvm/bin/llvm-tblgen', needed by 'include/llvm/IR/Attributes.inc.tmp', missing and no known rule to make it

when configured with configure --enable-rustbuild --host=$HOST or with configure --enable-rustbuild --host=$HOST --llvm-root=/usr.

I think this is because rustbuild tries to use the llvm-tblgen that's supposed to be generated by building LLVM for $BUILD but rusbuild nevers builds LLVM for $BUILD in this case.

cc @alexcrichton

[japaric]

Workaround

Call: x.py build --stage 1 src/llvm --host=$BUILD before x.py build --stage 1 src/rustc --host=$HOST.

[alexcrichton]

Fixed in #38050