Add -llvm-features parameter

With this is possible to enable e.g., SIMD in WASM using -llvm-features
+simd128.  Multiple features can be specified separated by comma,
e.g., -llvm-features +simd128,+tail-call

With help from @deadprogram and @aykevl.

main.go

@@ -911,6 +911,7 @@ func main() {
 	programmer := flag.String("programmer", "", "which hardware programmer to use")
 	ldflags := flag.String("ldflags", "", "Go link tool compatible ldflags")
 	wasmAbi := flag.String("wasm-abi", "", "WebAssembly ABI conventions: js (no i64 params) or generic")
+	llvmFeatures := flag.String("llvm-features", "", "comma separated LLVM features to enable")
 
 	var flagJSON, flagDeps *bool
 	if command == "help" || command == "list" {
@@ -978,6 +979,7 @@ func main() {
 		WasmAbi:         *wasmAbi,
 		Programmer:      *programmer,
 		OpenOCDCommands: ocdCommands,
+		LLVMFeatures:    *llvmFeatures,
 	}
 
 	os.Setenv("CC", "clang -target="+*target)