Fix x.py commands

This should make them faster, at least according to @jyn514.

Co-authored-by: Joshua Nelson <[email protected]>

Author: camelid

CONTRIBUTING.md

@@ -149,9 +149,9 @@ cp config.toml.example config.toml
 
 # Build a stage 2 rustc.
 # This step can take 30 minutes or more.
-./x.py build compiler/rustc --stage 2
+./x.py build --stage 2 library/std
 # If you change something, you can get a faster rebuild by doing
-./x.py --keep-stage 0 build compiler/rustc --stage 2
+./x.py build --keep-stage 0 --stage 2 library/std
 # You may have to change the architecture in the next command
 rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
 # Now cd to your Miri directory, then configure rustup