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.

compiler/compiler.go

@@ -59,6 +59,7 @@ type Config struct {
 	DefaultStackSize   uint64
 	NeedsStackObjects  bool
 	Debug              bool // Whether to emit debug information in the LLVM module.
+	LLVMFeatures       string
 }
 
 // compilerContext contains function-independent data that should still be
@@ -185,7 +186,12 @@ func NewTargetMachine(config *Config) (llvm.TargetMachine, error) {
 	if err != nil {
 		return llvm.TargetMachine{}, err
 	}
-	features := strings.Join(config.Features, ",")
+
+	feat := config.Features
+	if len(config.LLVMFeatures) > 0 {
+		feat = append(feat, config.LLVMFeatures)
+	}
+	features := strings.Join(feat, ",")
 
 	var codeModel llvm.CodeModel
 	var relocationModel llvm.RelocMode