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/arch_amd64.go

@@ -9,6 +9,8 @@ const deferExtraRegs = 0
 
 const callInstSize = 5 // "call someFunction" is 5 bytes
 
+const linux_MAP_ANONYMOUS = 0x20
+
 // Align a pointer.
 // Note that some amd64 instructions (like movaps) expect 16-byte aligned
 // memory, thus the result must be 16-byte aligned.