builder: use builtin Clang when building statically

This will be a huge help for people installing TinyGo that don't have
LLVM/Clang 9 already installed and in the $PATH variable.

builder/compiler-builtin.go

@@ -0,0 +1,56 @@
+// +build byollvm
+
+package builder
+
+import (
+	"errors"
+	"os"
+	"os/exec"
+	"unsafe"
+
+	"github.com/tinygo-org/tinygo/goenv"
+)
+
+/*
+#cgo CXXFLAGS: -fno-rtti
+#include <stdbool.h>
+#include <stdlib.h>
+bool tinygo_clang_driver(int argc, char **argv);
+*/
+import "C"
+
+// runCCompiler invokes a C compiler with the given arguments.
+//
+// This version invokes the built-in Clang when trying to run the Clang compiler.
+func runCCompiler(command string, flags ...string) error {
+	switch command {
+	case "clang":
+		// Compile this with the internal Clang compiler.
+		flags = append(flags, "-I"+getClangHeaderPath(goenv.Get("TINYGOROOT")))
+		flags = append([]string{"tinygo:" + command}, flags...)
+		var cflag *C.char
+		buf := C.calloc(C.size_t(len(flags)), C.size_t(unsafe.Sizeof(cflag)))
+		cflags := (*[1 << 10]*C.char)(unsafe.Pointer(buf))[:len(flags):len(flags)]
+		for i, flag := range flags {
+			cflag := C.CString(flag)
+			cflags[i] = cflag
+			defer C.free(unsafe.Pointer(cflag))
+		}
+		ok := C.tinygo_clang_driver(C.int(len(flags)), (**C.char)(buf))
+		if !ok {
+			return errors.New("failed to compile using built-in clang")
+		}
+		return nil
+	default:
+		// Running some other compiler. Maybe it has been defined in the
+		// commands map (unlikely).
+		if cmdNames, ok := commands[command]; ok {
+			return execCommand(cmdNames, flags...)
+		}
+		// Alternatively, run the compiler directly.
+		cmd := exec.Command(command, flags...)
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		return cmd.Run()
+	}
+}