Additions to gdb_mpz

Additions to gdb_mpz

In preparation for adding more 128-bit support to gdb, a few additions
to gdb_mpz are needed.

First, this adds a new 'as_integer_truncate' method.  This method
works like 'as_integer' but does not require the value to fit in the
target type -- it just truncates.

Second, gdb_mpz::export_bits is changed to handle the somewhat unusual
situation of zero-length types.  This can happen for a Rust '()' type;
but I think other languages have zero-bit integer types as well.

Finally, this adds some operator== overloads.