Build LLVM backend by default

Otherwise, the compiler can't actually compile anything.

Author: luca-barbieri

src/bootstrap/compile.rs

@@ -461,6 +461,7 @@ impl Step for Rustc {
 
 pub fn rustc_cargo(builder: &Builder<'_>, cargo: &mut Cargo, target: Interned<String>) {
     cargo
+        .arg("--no-default-features")
         .arg("--features")
         .arg(builder.rustc_features())
         .arg("--manifest-path")

src/rustc/Cargo.toml

@@ -21,5 +21,6 @@ optional = true
 features = ['unprefixed_malloc_on_supported_platforms']
 
 [features]
+default = ['llvm']
 jemalloc = ['jemalloc-sys']
 llvm = ['rustc_driver/llvm']