which will allow gm to work for more directories than just [<arch>].[<mode>]:
  gm.py ia32.release-nosnap.check
  gm.py x64.optdebug-ptr-compr cctest unittests

Basically the new usage is:
  gm.py [<arch>].[<mode>[-<suffix>]].[<target>] [testname...]

Once default gn configuration is created based on <arch> and <mode> the script user
may change it and then use gm as usual.

Bug: v8:8238
Change-Id: I9659b87073e815e0e4754f0a2f1056f3403c149c
Reviewed-on: https://chromium-review.googlesource.com/c/1323734Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57328}