Rewrite gdb_mpz::operator==

Simon pointed out that the recent changes to gdb_mpz caused a build
failure on amd64 macOS.  It turns out to be somewhat difficult to
overload a method in a way that will work "naturally" for all integer
types; especially in a case like gdb_mpz::operator==, where it's
desirable to special case all integer types that are no wider than
'long'.

After a false start, I came up with this patch, which seems to work.
It applies the desirable GMP special cases directly in the body,
rather than via overloads.

Approved-By: Simon Marchi <simon.marchi@efficios.com>