builder: run tools (clang, ...) as separate processes

This is necessary because LLVM defines many options in global variables
that are modified when invoking Clang. In particular, LLVM 10 seems to
have a bug in which it always sets the -pgo-warn-misexpect flag. Setting
it multiple times (over various cc1 invocations) results in an error:

    clang (LLVM option parsing): for the --pgo-warn-misexpect option: may only occur zero or one times!

This is fixed by running the Clang invocation in a new `tinygo`
invocation.

Because we've had issues with lld in the past, also run lld in a
separate process so similar issues won't happen with lld in the future.

main_test.go

@@ -6,6 +6,7 @@ package main
 import (
 	"bufio"
 	"bytes"
+	"fmt"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -16,6 +17,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/tinygo-org/tinygo/builder"
 	"github.com/tinygo-org/tinygo/compileopts"
 )
 
@@ -218,3 +220,24 @@ func runTest(path, target string, t *testing.T) {
 		t.Fail()
 	}
 }
+
+// This TestMain is necessary because TinyGo may also be invoked to run certain
+// LLVM tools in a separate process. Not capturing these invocations would lead
+// to recursive tests.
+func TestMain(m *testing.M) {
+	if len(os.Args) >= 2 {
+		switch os.Args[1] {
+		case "clang", "ld.lld", "wasm-ld":
+			// Invoke a specific tool.
+			err := builder.RunTool(os.Args[1], os.Args[2:]...)
+			if err != nil {
+				fmt.Fprintln(os.Stderr, err)
+				os.Exit(1)
+			}
+			os.Exit(0)
+		}
+	}
+
+	// Run normal tests.
+	os.Exit(m.Run())
+}