Left i16x8.extmul_low in the arch-specific macro-assemblers because
they rely on other functions defined in the same file. We can come back
and move it afterwards.

Bug: v8:11589
Change-Id: I2ea81c50ed52cc3e59e001b5e80aaf6b93a6572c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2786280Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73688}