Update the "[run-make] run-make/intrinsic-unreachable" test.

With rustc now emitting "ud2" on unreachable code, a "return nothing"
sequence may take the same number of lines as an "unreachable" sequence
in assembly code. This test is testing that intrinsics::unreachable()
works by testing that it reduces the number of lines in the assembly
code. Fix it by adding a return value, which requires an extra
instruction in the reachable case, which provides the test what it's
looking for.

src/test/run-make/intrinsic-unreachable/exit-ret.rs

@@ -11,10 +11,12 @@
 #![feature(asm)]
 #![crate_type="lib"]
 
-pub fn exit(n: usize) {
+#[deny(unreachable_code)]
+pub fn exit(n: usize) -> i32 {
     unsafe {
         // Pretend this asm is an exit() syscall.
         asm!("" :: "r"(n) :: "volatile");
         // Can't actually reach this point, but rustc doesn't know that.
     }
+    42
 }

src/test/run-make/intrinsic-unreachable/exit-unreachable.rs

@@ -13,10 +13,12 @@
 
 use std::intrinsics;
 
-pub fn exit(n: usize) -> ! {
+#[allow(unreachable_code)]
+pub fn exit(n: usize) -> i32 {
     unsafe {
         // Pretend this asm is an exit() syscall.
         asm!("" :: "r"(n) :: "volatile");
         intrinsics::unreachable()
     }
+    42
 }