all: add linux/mipsle support

This adds linux/mipsle (little endian Mips) support to TinyGo.

It also adds experimental linux/mips (big-endian) support. It doesn't
quite work yet, some parts of the standard library (like the reflect
package) currently seem to assume a little-endian system.

src/runtime/runtime_unix.go

@@ -229,6 +229,9 @@ func preinit() {
 			// heap size.
 			// This can happen on 32-bit systems.
 			heapMaxSize /= 2
+			if heapMaxSize < 4096 {
+				runtimePanic("cannot allocate heap memory")
+			}
 			continue
 		}
 		heapStart = uintptr(addr)