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.

compileopts/options.go

@@ -36,6 +36,7 @@ type Options struct {
 	TestConfig      TestConfig
 	Programmer      string
 	OpenOCDCommands []string
+	LLVMFeatures    string
 }
 
 // Verify performs a validation on the given options, raising an error if options are not valid.