Correct the arguments for std::fma() to double as the instruction expects
both addend and destination are 2*SEW bits wide. Addressed corresponding
implementation in the simulator as well.

R=qiuji@iscas.ac.cn

Change-Id: Ib3963a61c00ee9dc73af019574a1665de406cc32
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3878448
Commit-Queue: Yahan Lu <yahan@iscas.ac.cn>
Reviewed-by: Yahan Lu <yahan@iscas.ac.cn>
Cr-Commit-Position: refs/heads/main@{#83353}