When building without expat, we get a missing make_target_solib_ops
error:
/usr/bin/ld: arch-utils.o: in function `gdbarch::gdbarch()':
/home/simark/src/binutils-gdb/gdb/gdbarch-gen.c:30:(.text+0x15be): undefined reference to `make_target_solib_ops()'
Fix it by moving make_target_solib_ops out of HAVE_LIBEXPAT.