dns/doh-proxy: fix build on powerpc

  = note: ld: error: undefined symbol: __atomic_load_8
          >>> referenced by static.c
          >>>               98cfcaec7182b1d8-static.o:(mi_arena_schedule_purge) in archive /wrkdirs/usr/ports/dns/doh-proxy/work/rustc4xSzx7/liblibmimalloc_sys-4a9c093f9f8149a2.rlib
          >>> referenced by static.c
          >>>               98cfcaec7182b1d8-static.o:(mi_arenas_try_purge) in archive /wrkdirs/usr/ports/dns/doh-proxy/work/rustc4xSzx7/liblibmimalloc_sys-4a9c093f9f8149a2.rlib
          >>> referenced by static.c
          >>>               98cfcaec7182b1d8-static.o:(mi_stat_update) in archive /wrkdirs/usr/ports/dns/doh-proxy/work/rustc4xSzx7/liblibmimalloc_sys-4a9c093f9f8149a2.rlib
          >>> referenced 2 more times

          ld: error: undefined symbol: __atomic_fetch_add_8
          >>> referenced by static.c
          >>>               98cfcaec7182b1d8-static.o:(mi_arena_schedule_purge) in archive /wrkdirs/usr/ports/dns/doh-proxy/work/rustc4xSzx7/liblibmimalloc_sys-4a9c093f9f8149a2.rlib
          >>> referenced by static.c
          >>>               98cfcaec7182b1d8-static.o:(mi_manage_os_memory_ex2) in archive /wrkdirs/usr/ports/dns/doh-proxy/work/rustc4xSzx7/liblibmimalloc_sys-4a9c093f9f8149a2.rlib
          >>> referenced by static.c
          >>>               98cfcaec7182b1d8-static.o:(mi_manage_os_memory_ex2) in archive /wrkdirs/usr/ports/dns/doh-proxy/work/rustc4xSzx7/liblibmimalloc_sys-4a9c093f9f8149a2.rlib
          >>> referenced 88 more times

          ld: error: undefined symbol: __atomic_store_8
          >>> referenced by static.c
          >>>               98cfcaec7182b1d8-static.o:(mi_arena_schedule_purge) in archive /wrkdirs/usr/ports/dns/doh-proxy/work/rustc4xSzx7/liblibmimalloc_sys-4a9c093f9f8149a2.rlib

          ld: error: undefined symbol: __atomic_compare_exchange_8
          >>> referenced by static.c
          >>>               98cfcaec7182b1d8-static.o:(mi_arenas_try_purge) in archive /wrkdirs/usr/ports/dns/doh-proxy/work/rustc4xSzx7/liblibmimalloc_sys-4a9c093f9f8149a2.rlib
          >>> referenced by static.c
          >>>               98cfcaec7182b1d8-static.o:(mi_arenas_try_purge) in archive /wrkdirs/usr/ports/dns/doh-proxy/work/rustc4xSzx7/liblibmimalloc_sys-4a9c093f9f8149a2.rlib
          >>> referenced by static.c
          >>>               98cfcaec7182b1d8-static.o:(mi_stat_update) in archive /wrkdirs/usr/ports/dns/doh-proxy/work/rustc4xSzx7/liblibmimalloc_sys-4a9c093f9f8149a2.rlib

          ld: error: undefined symbol: __atomic_store
          >>> referenced by static.c
          >>>               98cfcaec7182b1d8-static.o:(mi_manage_os_memory_ex2) in archive /wrkdirs/usr/ports/dns/doh-proxy/work/rustc4xSzx7/liblibmimalloc_sys-4a9c093f9f8149a2.rlib
          cc: error: linker command failed with exit code 1 (use -v to see invocation)