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.

builder/tools-external.go

@@ -0,0 +1,15 @@
+// +build !byollvm
+
+package builder
+
+import "errors"
+
+const hasBuiltinTools = false
+
+// RunTool runs the given tool (such as clang).
+//
+// This version doesn't actually run the tool: TinyGo has not been compiled by
+// statically linking to LLVM.
+func RunTool(tool string, args ...string) error {
+	return errors.New("cannot run tool: " + tool)
+}