Fix spec non compliance by only trimming the unicode locales and not
all extensions.

Remove regexp and just use straightforward string manipulation.

Bug: v8:5751
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: Ie95828a8f62834daf8cde189f408e95a14e796fe
Reviewed-on: https://chromium-review.googlesource.com/c/1255556
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56458}