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/asm_mipsx.S

@@ -0,0 +1,40 @@
+.section .text.tinygo_scanCurrentStack
+.global  tinygo_scanCurrentStack
+.type    tinygo_scanCurrentStack, %function
+tinygo_scanCurrentStack:
+    // Push callee-saved registers onto the stack.
+    addiu $sp, $sp, -40
+    sw $ra, 36($sp)
+    sw $s8, 32($sp)
+    sw $s7, 28($sp)
+    sw $s6, 24($sp)
+    sw $s5, 20($sp)
+    sw $s4, 16($sp)
+    sw $s3, 12($sp)
+    sw $s2,  8($sp)
+    sw $s1,  4($sp)
+    sw $s0,   ($sp)
+
+    // Scan the stack.
+    jal tinygo_scanstack
+    move $a0, $sp // in the branch delay slot
+
+    // Restore return address.
+    lw $ra, 36($sp)
+
+    // Restore stack state.
+    addiu $sp, $sp, 40
+
+    // Return to the caller.
+    jalr $ra
+    nop
+
+.section .text.tinygo_longjmp
+.global tinygo_longjmp
+tinygo_longjmp:
+    // Note: the code we jump to assumes a0 is non-zero, which is already the
+    // case because that's the defer frame pointer.
+    lw $sp, 0($a0) // jumpSP
+    lw $a1, 4($a0) // jumpPC
+    jr $a1
+    nop