emulators/mame: add MAKE_JOBS_UNSAFE=yes

There's a race condition when compiling with 18-core POWER9 (72 threads):
In file included from ../../../../../src/mame/drivers/konamigq.cpp:76:
../../../../../src/devices/cpu/tms57002/tms57002.h:208:10: fatal error: '../../emu/cpu/tms57002/tms57002.hxx' file not found
#include "../../emu/cpu/tms57002/tms57002.hxx"

^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~