mips: fix a bug when scanning the stack

Previously the assembler was reordering this code:

    jal tinygo_scanstack
    move $a0, $sp

Into this:

    jal tinygo_scanstack
    nop
    move $a0, $sp

So it was "helpfully" inserting a branch delay slot, even though this
was already being taken care of.
Somehow this didn't break, but it does break in the WIP threading branch
(#4559) where this bug leads to
a crash.

Author: aykevl

src/runtime/asm_mipsx.S

@@ -1,3 +1,7 @@
+// Do not reorder instructions to insert a branch delay slot.
+// We know what we're doing, and will manually fill the branch delay slot.
+.set noreorder
+
 .section .text.tinygo_scanCurrentStack
 .global  tinygo_scanCurrentStack
 .type    tinygo_scanCurrentStack, %function