Fully define all the registers needed on mips.

Index: js/src/wasm/WasmSignalHandlers.cpp
--- js/src/wasm/WasmSignalHandlers.cpp.orig
+++ js/src/wasm/WasmSignalHandlers.cpp
@@ -105,6 +105,8 @@ using mozilla::DebugOnly;
 #    if defined(__mips__)
 #      define EPC_sig(p) ((p)->sc_pc)
 #      define RFP_sig(p) ((p)->sc_regs[30])
+#      define RSP_sig(p) ((p)->sc_regs[29])
+#      define R31_sig(p) ((p)->sc_regs[31])
 #    endif
 #    if defined(__ppc64__) || defined(__PPC64__) || defined(__ppc64le__) || \
         defined(__PPC64LE__)
