Fix runtime build on current nightly

libstd’s runtime changed the prototype of lang = "start" in
ddee45e1d7fd34563c13513d974f792fae41a2f7 to support changing the SIGPIPE
behaviour on Linux, so this function now takes a third argument on every
platform.

Additionally, it now supports user_main() returning any Termination, so
we don’t need a transmute() any longer. :)

luma_runtime/src/lib.rs

@@ -35,7 +35,7 @@ global_asm!(include_str!("../asm/system.S"));
 /// This is the executable start function, which directly follows the entry point.
 #[cfg_attr(not(test), lang = "start")]
 #[cfg(not(test))]
-fn start<T>(user_main: fn(), _argc: isize, _argv: *const *const u8) -> isize
+fn start<T>(user_main: fn() -> T, _argc: isize, _argv: *const *const u8, _sigpipe: u8) -> isize
 where
     T: Termination,
 {
@@ -54,7 +54,6 @@ where
     }
 
     // Jump to user defined main function.
-    let user_main: fn() -> T = unsafe { core::mem::transmute(user_main) };
     user_main();
 
     panic!("main() cannot return");