Fix FTBFS on power "not a constant expression"There are some explanation upstream : https://github.com/mamedev/mame/issues/3157 and this probably due to the fact that IBM 128bit long double format is not constant folded. I slighlty rewrote ""_kHz_XTAL(long double clock) and ""_MHz_XTAL(long double clock) the way ""_kHz_XTAL(unsigned long long clock) and ""_MHz_XTAL(unsigned long long clock) which makes the compiler happy. Also including an upstream change about rounding to get same results as on x86 : https://github.com/mamedev/mame/pull/5164