[flang] Make real type of kind 10 target dependent

The real(10) is supported on x86_64. On aarch64, the value of
selected_real_kind(16) should be 16 rather than 10 since real(10)
is not supported on x86_64. Previously, the real type support check
is not target dependent. Support it now through the target triple
information.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D134021