[tools][gm] Fix aliases for modes with a suffix

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}

tools/dev/gm.py

@@ -484,11 +484,16 @@ class ArgumentParser(object):
         targets.append(word)
       elif word in ACTIONS:
         actions.append(word)
-      elif any(word.startswith(mode + "-") for mode in MODES.keys()):
-        modes.append(MODES[word])
       else:
-        print("Didn't understand: %s" % word)
-        sys.exit(1)
+        for mode in MODES.keys():
+          if word.startswith(mode + "-"):
+            prefix = word[:len(mode)]
+            suffix = word[len(mode) + 1:]
+            modes.append(MODES[prefix] + "-" + suffix)
+            break
+        else:
+          print("Didn't understand: %s" % word)
+          sys.exit(1)
     # Process actions.
     for action in actions:
       impact = ACTIONS[action]