Don't use a strict larger-than comparison in the check_fit/does_fit static assert

For builds that only target one architecture, this was required to
be an exact match, while it previously required the allocation to be
strictly larger than the largest concrete one. Requiring it to be
larger than on equal should be enough.

This makes it more straightforward to update _LIBUNWIND_CONTEXT_SIZE
and _LIBUNWIND_CURSOR_SIZE.

llvm-svn: 321679