Most function signatures are created once and never changed. Hence pass
them as const pointer. This makes it clear in function signatures that
these parameters will not be modified.

This also avoids a few ugly const_casts where we were passing pointers
to constexpr FunctionSigs via non-const pointers.

R=jkummerow@chromium.org

Bug: v8:10155
Change-Id: Ieb658ab5582bff276f76babdaf7ddb8f72bd4790
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2072739Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66478}