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.

builder/build.go

@@ -94,6 +94,7 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 		DefaultStackSize:   config.Target.DefaultStackSize,
 		NeedsStackObjects:  config.NeedsStackObjects(),
 		Debug:              config.Debug(),
+		LLVMFeatures:       config.LLVMFeatures(),
 	}
 
 	// Load the target machine, which is the LLVM object that contains all