rustbuild: target.*.musl-root in config.toml is ignored

[japaric]

AFAICT, the CFG_MUSL_ROOT_X86_64 "variable" in the config.mk file that configure generates overrides the target.x86_64-unknown-linux-musl.musl-root setting in config.toml. I think the precedence should be the other way around.

cc @alexcrichton is CFG_MUSL_ROOT_X86_64 new-ish?

[alexcrichton]

Ah yes that's newer and should be preferred. I think I ran into this awhile ago as well.

I think there's a bug in rustbuild to ignore the x86_64 default if musl-root is specified, but we could also just stop documenting musl-root in rustbuild.