[mips64] Temporarily add s0 to scratch register list

Debug build needs more scratch registers, since s0 is not added to
allocatable registers, we could use it as a scratch register.

Change-Id: I1453d78b18cd3abff83a2f0c084ed4dd92e92884
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3313967Reviewed-by: Liu yu <liuyu@loongson.cn>
Commit-Queue: Liu yu <liuyu@loongson.cn>
Cr-Commit-Position: refs/heads/main@{#78225}

src/codegen/mips64/assembler-mips64.cc

@@ -265,7 +265,7 @@ const Instr kLwSwOffsetMask = kImm16Mask;
 Assembler::Assembler(const AssemblerOptions& options,
                      std::unique_ptr<AssemblerBuffer> buffer)
     : AssemblerBase(options, std::move(buffer)),
-      scratch_register_list_(at.bit()) {
+      scratch_register_list_(at.bit() | s0.bit()) {
   if (CpuFeatures::IsSupported(MIPS_SIMD)) {
     EnableCpuFeature(MIPS_SIMD);
   }