z3: restrict to x86_64

it uses x86-only header:

include <immintrin.h>
          ^~~~~~~~~~~~~
compilation terminated.
src/sat/sat_probing.cpp
make: *** [Makefile:182: util/mpz.o] Error 1
make: *** Waiting for unfinished jobs....
src/sat/sat_parallel.cpp
builder for '/nix/store/vd2wkhic8g77izxv659ackh6hcaamic3-z3-4.8.1.drv' failed with exit code 2
cannot build derivation '/nix/store/h5imnjsf31c45l558gw66vyzb0ickc1m-solc-0.5.0.drv': 1 dependencies couldn't be built
error: build of '/nix/store/h5imnjsf31c45l558gw66vyzb0ickc1m-solc-0.5.0.drv' failed