testing/rethinkdb: disable on arm

* Warning: ARM support is still experimental

... and it fails with:

In file included from ./src/containers/printf_buffer.hpp:11:0,
                 from ./src/containers/archive/archive.hpp:10,
                 from ./src/rdb_protocol/serialize_datum.hpp:6,
                 from src/rdb_protocol/serialize_datum.cc:2:
./src/errors.hpp: In function 'size_t ql::datum_get_element_offset(const shared_buf_ref_t<char>&, size_t)':
./src/errors.hpp:122:9: error: 'element_offset' may be used uninitialized in this function [-Werror=maybe-uninitialized]
         if (!(cond)) {                  \
         ^
src/rdb_protocol/serialize_datum.cc:939:18: note: 'element_offset' was declared here
         uint64_t element_offset;
                  ^
    [155/400] CC build/release_system/obj/rdb_protocol/artificial_table/caching_cfeed_backend.o
cc1plus: all warnings being treated as errors
src/build.mk:381: recipe for target 'build/release_system/obj/rdb_protocol/serialize_datum.o' failed

0 files changed, 0 insertions, 0 deletions