tell emcc stip exception handling if panic rt used

src/librustc_back/target/asmjs_unknown_emscripten.rs

@@ -22,6 +22,7 @@ pub fn target() -> Result<Target, String> {
         linker_is_gnu: true,
         allow_asm: false,
         obj_is_bitcode: true,
+        is_like_emscripten: true,
         max_atomic_width: Some(32),
         post_link_args: vec!["-s".to_string(), "ERROR_ON_UNDEFINED_SYMBOLS=1".to_string()],
         target_family: Some("unix".to_string()),

src/librustc_back/target/mod.rs

@@ -330,6 +330,10 @@ pub struct TargetOptions {
     /// Whether the target toolchain is like Android's. Only useful for compiling against Android.
     /// Defaults to false.
     pub is_like_android: bool,
+    /// Whether the target toolchain is like Emscripten's. Only useful for compiling with
+    /// Emscripten toolchain.
+    /// Defaults to false.
+    pub is_like_emscripten: bool,
     /// Whether the linker support GNU-like arguments such as -O. Defaults to false.
     pub linker_is_gnu: bool,
     /// The MinGW toolchain has a known issue that prevents it from correctly
@@ -428,6 +432,7 @@ impl Default for TargetOptions {
             is_like_solaris: false,
             is_like_windows: false,
             is_like_android: false,
+            is_like_emscripten: false,
             is_like_msvc: false,
             linker_is_gnu: false,
             allows_weak_linkage: true,

src/librustc_back/target/wasm32_unknown_emscripten.rs

@@ -24,6 +24,7 @@ pub fn target() -> Result<Target, String> {
         linker_is_gnu: true,
         allow_asm: false,
         obj_is_bitcode: true,
+        is_like_emscripten: true,
         max_atomic_width: Some(32),
         post_link_args: vec!["-s".to_string(), "BINARYEN=1".to_string(),
                              "-s".to_string(), "ERROR_ON_UNDEFINED_SYMBOLS=1".to_string()],