fortran: map atand(y, x) to atan2d(y, x) [PR113413]

fortran: map atand(y, x) to atan2d(y, x) [PR113413]

According to the Fortran standard, atand(y, x) is equivalent to atan2d(y, x).
However, the current atand(y, x) function produces an error. This patch
includes the necessary intrinsic mapping, related test, and intrinsic
documentation.
The minor comment change in intrinsic.cc is cherry-picked from Steve's previous
work.

PR fortran/113413 - ATAND(Y,X) is unsupported

	PR fortran/113413

gcc/fortran/ChangeLog:

	* intrinsic.cc (do_check): Minor doc polish.
	(add_functions): Add atand(y, x) mapping.
	* intrinsic.texi: Update atand example.

gcc/testsuite/ChangeLog:

	* gfortran.dg/dec_math.f90: Add atand(y, x) testcase.

Signed-off-by: Yuao Ma <c8ef@outlook.com>
Co-authored-by: Steven G. Kargl <kargl@gcc.gnu.org>