The t6-t8 are scratch registers and should not be allocatable.

Besides, add s0, s1, s2, s5 and s8 as allocatable registers.

Change-Id: I0805cc5273d0e0ec5040a0376bcbfba276202077
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3147315
Commit-Queue: Zhao Jiazhong <zhaojiazhong-hf@loongson.cn>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76739}