mips: add native backtrace implementation

Should make debugging crashes on native MIPS targets far easier.
This is by no means a 100% complete or robust implementation but
it seems to handle the vast majority of functions.

Change-Id: Id5f430270e02b5092b79026b6876675c784aa649

lib/mipsunwinder/init_context_32.S

@@ -0,0 +1,12 @@
+#include "mips.h"
+
+    .text
+    .global mips_bt_start
+
+mips_bt_start:
+    addiu   v0, ra, -8
+    sw      v0, 0(a0)       /* ctx->pc = ra - 8 */
+    sw      sp, 4(a0)       /* ctx->sp = sp */
+    sw      zero, 8(a0)     /* ctx->depth = 0 */
+    sw      zero, 12(a0)    /* ctx->valid = 0 */
+    jr      ra