With current OpenBSD, code_gen_buffer was mapped 8GB away from
text segment. Then any helpers were beyond the 2GB range of call
instruction genereated by TCG and so the calls would go nowhere,
leading to a segfault.

Fix by specifying an address for the code_gen_buffer,
hopefully free and nearby the helpers.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>