Open code the __failfast intrinsic for rtabort! on windows

As described https://msdn.microsoft.com/en-us/library/dn774154.aspx

This is a Windows 8+ mechanism for terminating the process quickly,
which degrades to either an access violation or bugcheck in older versions.

I'm not sure this is better the the current mechanism of terminating
with an illegal instruction, but we recently converted unix to
terminate more correctly with SIGABORT, and this *seems* more correct
for windows.

[breaking-change]

Author: brson

src/libstd/sys/common/util.rs

@@ -40,11 +40,24 @@ pub fn abort(args: fmt::Arguments) -> ! {
     unsafe { libc::abort(); }
 }
 
-#[cfg(not(unix))]
+// On windows, use the processor-specific __fastfail mechanism
+// https://msdn.microsoft.com/en-us/library/dn774154.aspx
+#[cfg(all(windows, target_arch = "x86"))]
 pub fn abort(args: fmt::Arguments) -> ! {
-    use intrinsics;
     dumb_print(format_args!("fatal runtime error: {}\n", args));
-    unsafe { intrinsics::abort(); }
+    unsafe {
+        asm!("int $$0x29" :: "{ecx}"(7) ::: volatile); // 7 is FAST_FAIL_FATAL_APP_EXIT
+        ::intrinsics::unreachable();
+    }
+}
+
+#[cfg(all(windows, target_arch = "x86_64"))]
+pub fn abort(args: fmt::Arguments) -> ! {
+    dumb_print(format_args!("fatal runtime error: {}\n", args));
+    unsafe {
+        asm!("int $$0x29" :: "{rcx}"(7) ::: volatile); // 7 is FAST_FAIL_FATAL_APP_EXIT
+        ::intrinsics::unreachable();
+    }
 }
 
 #[allow(dead_code)] // stack overflow detection not enabled on all platforms