On powerpc64 the long double type is special in that it does not permit
a constexpr return type for a function where a long double constant and
a double constant are multiplied. Yet this is what the MAME _MHz_XTAL and
_GHz_XTAL constexpr suffixes on numbers expect from the multiplication
of a long double value with 1e6 or 1e9 respectively.
To make the code compile, knowing it will produce differing results for
e.g. 6'144'000_Hz_XTAL and 6.144_MHz_XTAL, cast the long double constant
down to double before multiplying by 1e6 or 1e9.
Do this iff the preprocessor macro _GLIBCXX_LONG_DOUBLE_COMPAT is defined.
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