Minor bug fix in alias support (crrev.com/c/3723506), which broke
modes with suffixes, e.g. x64.release-css or x64.rel-css

No-Try: True
Change-Id: I16fdc83dde269f66f4bb7260de0d2649aaece27e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3732929
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81541}