Implement `eh_unwind_resume` in libstd.

src/libstd/sys/common/libunwind.rs

@@ -142,6 +142,9 @@ extern "C" {
                                    -> _Unwind_Reason_Code;
 
     pub fn _Unwind_DeleteException(exception: *mut _Unwind_Exception);
+
+    #[unwind]
+    pub fn _Unwind_Resume(exception: *mut _Unwind_Exception) -> !;
 }
 
 // ... and now we just providing access to SjLj counterspart

src/libstd/sys/common/unwind/gcc.rs

@@ -231,3 +231,10 @@ pub mod eabi {
         }
     }
 }
+
+#[cfg(all(target_os="windows", target_arch = "x86", target_env="gnu", not(test)))]
+#[lang = "eh_unwind_resume"]
+#[unwind]
+unsafe extern fn rust_eh_unwind_resume(panic_ctx: *mut u8) -> ! {
+    uw::_Unwind_Resume(panic_ctx as *mut uw::_Unwind_Exception);
+}

src/libstd/sys/common/unwind/seh64_gnu.rs

@@ -190,17 +190,10 @@ unsafe extern fn rust_eh_personality(
     ExceptionContinueSearch
 }
 
-// The `resume` instruction, found at the end of the landing pads, and whose job
-// is to resume stack unwinding, is typically lowered by LLVM into a call to
-// `_Unwind_Resume` routine.  To avoid confusion with the same symbol exported
-// from libgcc, we redirect it to `rust_eh_unwind_resume`.
-// Since resolution of this symbol is done by the linker, `rust_eh_unwind_resume`
-// must be marked `pub` + `#[no_mangle]`.  (Can we make it a lang item?)
-
-#[lang = "eh_unwind_resume"]
 #[cfg(not(test))]
+#[lang = "eh_unwind_resume"]
 #[unwind]
-unsafe extern fn rust_eh_unwind_resume(panic_ctx: LPVOID) {
+unsafe extern fn rust_eh_unwind_resume(panic_ctx: LPVOID) -> ! {
     let params = [panic_ctx as ULONG_PTR];
     RaiseException(RUST_PANIC,
                    EXCEPTION_NONCONTINUABLE,