rustc: Compile the fmt_macros crate as an rlib

I think this was left out by accident from the "convert everything to
rlibs" commit, there's no need for this to be a dylib just as everything
else doesn't need to be a dylib!

src/libfmt_macros/Cargo.toml

@@ -7,7 +7,6 @@ edition = "2018"
 [lib]
 name = "fmt_macros"
 path = "lib.rs"
-crate-type = ["dylib"]
 
 [dependencies]
 syntax_pos = { path = "../libsyntax_pos" }