Re-enable Emscripten's exception handling support

Passes LLVM codegen and Emscripten link-time flags for exception
handling if and only if the panic strategy is `unwind`. Sets the
default panic strategy for Emscripten targets to `unwind`. Re-enables
tests that depend on unwinding support for Emscripten, including
`should_panic` tests.

src/test/ui/iterators/iter-sum-overflow-overflow-checks.rs

@@ -1,5 +1,5 @@
 // run-pass
-// ignore-emscripten compiled with panic=abort by default
+// ignore-wasm32-bare compiled with panic=abort by default
 // compile-flags: -C overflow-checks
 
 use std::panic;