main: optionally build with LLD

When building statically against LLVM, LLD is also included now. When
included, the built in wasm-ld will automatically be used instead of the
external command.

There is also support for linking ELF files but because lld does not
fully support armv6m this is not yet enabled (it produces a warning).

linker-external.go

@@ -0,0 +1,22 @@
+// +build !byollvm
+
+package main
+
+// This file provides a Link() function that always runs an external command. It
+// is provided for when tinygo is built without linking to liblld.
+
+import (
+	"os"
+	"os/exec"
+)
+
+// Link invokes a linker with the given name and arguments.
+//
+// This version always runs the linker as an external command.
+func Link(dir, linker string, flags ...string) error {
+	cmd := exec.Command(linker, flags...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.Dir = dir
+	return cmd.Run()
+}