bootstrap: dont require cmake if local-rebuild is enabled

This is for people rebuilding stdlib directly from stage 0
with the full toolchain from rust-src rustup component.
The toolchain itself should have sufficient LLVM tools,
so CMake and LLVM are not required when `build.local-rebuild = true`

src/bootstrap/src/core/sanity.rs

@@ -142,6 +142,7 @@ pub fn check(build: &mut Build) {
 
     // We need cmake, but only if we're actually building LLVM or sanitizers.
     let building_llvm = !build.config.llvm_from_ci
+        && !build.config.local_rebuild
         && build.hosts.iter().any(|host| {
             build.config.llvm_enabled(*host)
                 && build