Fortran: allow __float128 on targets where long double is not REAL(KIND=10)

Fortran: allow __float128 on targets where long double is not REAL(KIND=10)

The logic for detection of REAL(KIND=16) in kinds-override.h made
assumptions:

    -- if real(kind=10) exists, i.e. if HAVE_GFC_REAL_10 is defined,
       then it is necessarily the "long double" type
    -- if real(kind=16) exists, then:
       * if HAVE_GFC_REAL_10, real(kind=16) is "__float128"
       * otherwise, real(kind=16) is "long double"

This may not always be true. Take the aarch64-apple-darwin port,
it has double == long double == binary64, and __float128 == binary128.

We already have more fine-grained logic in the mk-kinds-h.sh script,
where we actually check the Fortran kind corresponding to C’s long
double. So let's use it, and emit the GFC_REAL_16_IS_FLOAT128 /
GFC_REAL_16_IS_LONG_DOUBLE macros there.

libgfortran/ChangeLog:

	* kinds-override.h: Move GFC_REAL_16_IS_* macros...
	* mk-kinds-h.sh: ... here.