remove -fstage1 option

After this commit, the self-hosted compiler does not offer the option to
use stage1 as a backend anymore.

test/link/wasm/stack_pointer/build.zig

@@ -11,7 +11,6 @@ pub fn build(b: *Builder) void {
     lib.setBuildMode(mode);
     lib.setTarget(.{ .cpu_arch = .wasm32, .os_tag = .freestanding });
     lib.use_llvm = false;
-    lib.use_stage1 = false;
     lib.use_lld = false;
     lib.strip = false;
     lib.stack_size = std.wasm.page_size * 2; // set an explicit stack size