[Bug 38713] Link failure due to use of libunwind in ntdll

https://bugs.winehq.org/show_bug.cgi?id=38713

--- Comment #13 from Ken Thomases  ---
It's true that it's (currently) only used for 64-bit, but I'm guessing it's not
the first such architecture-specific dependency.  I'm not sure it's worth
making the check architecture-specific if there's any chance that it will be
used for other architectures in the future (which I don't really know if it
will).

Your patch is better than mine in that I forgot to #define UNW_LOCAL_ONLY for
my check.

The loop construct in mine avoids the duplication of the test code in yours.