Fix rustc binary metadata overwriting librustc metadata

In #49289, rustc was changed to emit metadata for binaries, which made
it so that the librustc.rmeta file created when compiling librustc was
overwritten by the rustc-main compilation. This commit renames the
rustc-main binary to avoid this problem.

rust-lang/cargo#5524 has also been filed to
see if Cargo can learn to warn on this situation instead of leaving it
for the user to debug.

src/bootstrap/compile.rs

@@ -972,7 +972,7 @@ impl Step for Assemble {
 
         // Link the compiler binary itself into place
         let out_dir = builder.cargo_out(build_compiler, Mode::Librustc, host);
-        let rustc = out_dir.join(exe("rustc", &*host));
+        let rustc = out_dir.join(exe("rustc_binary", &*host));
         let bindir = sysroot.join("bin");
         t!(fs::create_dir_all(&bindir));
         let compiler = builder.rustc(target_compiler);

src/rustc/Cargo.toml

@@ -4,7 +4,7 @@ name = "rustc-main"
 version = "0.0.0"
 
 [[bin]]
-name = "rustc"
+name = "rustc_binary"
 path = "rustc.rs"
 
 [dependencies]