Allow cast of 128-bit integer to pointer

Allow cast of 128-bit integer to pointer

PR rust/31082 points out that casting a 128-bit integer to a pointer
will fail.  This happens because a case in value_cast was not
converted to use GMP.

This patch fixes the problem.  I am not really sure that testing
against the negative value here makes sense, but I opted to just
preserve the existing behavior rather than change it.

Regression tested on x86-64 Fedora 38.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31082