It's better to avoid references to this global variable, but, in the
cases where it's needed, such as in std.debug.print and collecting stack
traces, better to share the same instance.
Also add a standalone test which covers the `-fentry` case. It does this
by performing two reproducible compilations which are identical other
than having different entry points, and checking whether the emitted
binaries are identical (they should *not* be).
Resolves: #23869
Andrew Kelley