PR fortran/28484
	PR fortran/61429

	* check.c (gfc_check_system_clock): Improve checking of arguments.
	* intrinsic.texi: Update doc of SYSTEM_CLOCK.
	* iresolve.c (gfc_resolve_system_clock): Choose library function
	used depending on argument kinds.
	* trans-decl.c (gfc_build_intrinsic_function_decls): Build
	decls for system_clock_4 and system_clock_8.
	* trans-intrinsic.c (conv_intrinsic_system_clock): New function.
	(gfc_conv_intrinsic_subroutine): Call conv_intrinsic_system_clock.
	* trans.h (gfor_fndecl_system_clock4, gfor_fndecl_system_clock8):
	New variables.

	* gfortran.dg/system_clock_1.f90: New file.
	* gfortran.dg/system_clock_2.f90: New file.

From-SVN: r211686