emulators/mame: fix build on powerpc64

powerpc64 requires the same Makefile fix as amd64.

Additionally, https://github.com/bkaradzic/bx/commit/684f7ec5d6a8e0d9c396bdd2dfc1f19244800557 is necessary.

Approved by: linimon (mentor)
Differential Revision: https://reviews.freebsd.org/D21055