[flang] Fold transformational bessels when host runtime has bessels

Transformational bessel intrinsic functions require the same math runtime
as elemental bessel intrinsics.

Currently elemental bessels could be folded if f18 was linked with pgmath
(cmake -DLIBPGMATH_DIR option). `j0`, `y0`, ... C libm functions were not
used because they are not standard C functions: they are Posix
extensions.

This patch enable:
- Using the Posix bessel host runtime functions when available.
- folding the transformational bessel using the elemental version.

Differential Revision: https://reviews.llvm.org/D124167