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).

lld.cpp

@@ -0,0 +1,19 @@
+// +build byollvm
+
+// This file provides C wrappers for liblld.
+
+#include <lld/Common/Driver.h>
+
+extern "C" {
+
+bool tinygo_link_elf(int argc, char **argv) {
+	std::vector<const char*> args(argv, argv + argc);
+	return lld::elf::link(args, false);
+}
+
+bool tinygo_link_wasm(int argc, char **argv) {
+	std::vector<const char*> args(argv, argv + argc);
+	return lld::wasm::link(args, false);
+}
+
+} // external "C"