When compiling on aarch64 some `LDBL_MANT_DIG == 113` entries
end up trying to use `complex<long double>` for which there are
no certain specializations in `libcudacxx`. This change-set
includes a clean-up for `LDBL_MANT_DIG == 113` usage, which is replaced
with `HAS_LDBL128` that is set in `float128.h`.
For `LDBL_MANT_DIG == 113` targets the FortranFloat128Math library
is just an interface library that provides sources and compilation
options to be used for building FortranRuntime - there are not extra
dependencies on other libraries, so it can be a part of FortranRuntime,
which helps to avoid extra linking steps in the compiler driver.
Targets with __float128 support in libc will also use this path.
Other targets, where the math support comes from
FLANG_RUNTIME_F128_MATH_LIB,
FortranFloat128Math is built as a standalone static library,
and the compiler driver needs to conduct the linking.
Flang APIs for COMPLEX(16) are just thin C wrappers around
the C math functions. Flang uses C _Complex ABI for passing/returning
COMPLEX values, so the runtime is aligned to this.
