Remove unnecessary flags

The default-linker-libraries flag requires a change in the rustc target
to set `no_default_libraries` to false.

The muldefs flag isn't necessary anymore. I think the removal of static
libs (ex. linker fix and pthread_3ds) might have fixed this issue.

The __3DS__ flag doesn't seem to have been necessary at all.

Similarly, `-Z unstable-options` isn't needed for cargo.

src/main.rs

@@ -109,14 +109,11 @@ fn check_rust_version() {
 }
 
 fn build_elf(args: std::iter::Skip<env::Args>) {
-    let rustflags = env::var("RUSTFLAGS").unwrap_or("".into())
-    + "-C default-linker-libraries -Clink-arg=-z -Clink-arg=muldefs -Clink-arg=-D__3DS__";
+    let rustflags = env::var("RUSTFLAGS").unwrap_or_default();
 
     let mut process = Command::new("cargo")
         .arg("build")
         .arg("-Z")
-        .arg("unstable-options")
-        .arg("-Z")
         .arg("build-std")
         .arg("--target")
         .arg("armv6k-nintendo-3ds")