Torque generally supports marking things as unused by adding a single
underscore to the front of identifiers. For locals, this feature works
fine. For macros, it's only half-implemented: attempting to look up a
macro that starts with _ fails (as it should), but the compiler also
complains if the macro is unused. This change avoids emitting the latter
error.

Bug: v8:7793
Change-Id: Ib021c053004a180dd31993d3ad06cde463bafd5d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2757208
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73397}