VEX/auxprogs/genoffsets.c: Add cast to my_offsetof. n-i-bz.

VEX/auxprogs/genoffsets.c: Add cast to my_offsetof.  n-i-bz.

Clang/LLVM trips over my_offsetof in VEX/auxprogs/genoffsets.c.  See LLVM
PR 40890 for details (https://bugs.llvm.org/show_bug.cgi?id=40890).

Now, it's a Clang bug that Clang exits on an assertion failure rather than
emits a diagnostic, but the previous my_offsetof expression is a pointer,
not an integer.  Add a cast as done in other definitions of offsetof in
the tree.  Patch from Ed Maste <emaste@freebsd.org>.