Don't use static crt by default when build proc-macro.

src/test/ui/proc-macro/musl-proc-macro.rs

@@ -0,0 +1,15 @@
+// Test proc-macro crate can be built without addtional RUSTFLAGS
+// on musl target
+
+// run-pass
+// compile-flags: --target=x86_64-unknown-linux-musl
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+#[proc_macro_derive(Foo)]
+pub fn derive_foo(input: TokenStream) -> TokenStream {
+    input
+}