[flang][runtime] Fixed identity value for REAL(16) == __float128.

std::numeric_limits<__float128>::max/lowest return 0.0, so recreate
value of FLT128_MAX ourselves to avoid using quadmath.h's FLT128_MAX
that is currently causes warnings with GCC -Wpedantic.

Differential Revision: https://reviews.llvm.org/D134496