[riscv64] add a7 into wasm-linkage

and delete extra asm_comment

Change-Id: Ia22f4bc622387e7c9a1c830b9f213d9554f0029e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3507121


Reviewed-by: Clemens Backes <clemensb@chromium.org>
Auto-Submit: Yahan Lu <yahan@iscas.ac.cn>
Commit-Queue: Yahan Lu <yahan@iscas.ac.cn>
Cr-Commit-Position: refs/heads/main@{#79401}