[flang] Fixed the declarations of IEEE_SUPPORT_FLAG and IEEE_SUPPORT_HALTING to

be functions rather than subroutines.  Note that Table 17.3 in the
standard summarizes the contents of IEEE_EXCEPTIONS with the details
specified in section 17.11.

Original-commit: flang-compiler/f18@fca58d479d2ab4578a029980f5144d11a46d5e78
Reviewed-on: https://github.com/flang-compiler/f18/pull/730