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_mips.go

@@ -0,0 +1,21 @@
+package runtime
+
+const GOARCH = "mips"
+
+// The bitness of the CPU (e.g. 8, 32, 64).
+const TargetBits = 32
+
+const deferExtraRegs = 0
+
+const callInstSize = 8 // "jal someFunc" is 4 bytes, plus a MIPS delay slot
+
+const linux_MAP_ANONYMOUS = 0x800
+
+// It appears that MIPS has a maximum alignment of 8 bytes.
+func align(ptr uintptr) uintptr {
+	return (ptr + 7) &^ 7
+}
+
+func getCurrentStackPointer() uintptr {
+	return uintptr(stacksave())
+}