stage2: add zig_backend to the cache hash

This makes stage2 and stage3 have different cache namespaces, so that
building something with stage3 does not try to reuse the same cached
artifacts as were produced by stage2. This makes sense since the code
of stage3 is produced by the self-hosted compiler, whereas the code of
stage2 is produced by the bootstrap compiler. Note also that stage4 and
stage3 will share the same zig_backend, end hence cache namespace.
Ideally stage4 and stage3 are identical binaries, so this checks out.

src/Compilation.zig

@@ -1224,6 +1224,7 @@ pub fn create(gpa: Allocator, options: InitOptions) !*Compilation {
 
         // This is shared hasher state common to zig source and all C source files.
         cache.hash.addBytes(build_options.version);
+        cache.hash.add(builtin.zig_backend);
         cache.hash.addBytes(options.zig_lib_directory.path orelse ".");
         cache.hash.add(options.optimize_mode);
         cache.hash.add(options.target.cpu.arch);