runtime (gc): add garbage collector that uses an external allocator

src/runtime/runtime_unix_heap.go

@@ -0,0 +1,14 @@
+// +build darwin linux,!baremetal freebsd,!baremetal
+
+// +build gc.conservative gc.leaking
+
+package runtime
+
+const heapSize = 1 * 1024 * 1024 // 1MB to start
+
+var heapStart, heapEnd uintptr
+
+func preinit() {
+	heapStart = uintptr(malloc(heapSize))
+	heapEnd = heapStart + heapSize
+}