libfortran: Fix build for targets that don't have 10byte or 16 byte floating point
So the problem here is EXPAND_INTER_MACRO_16 expands to nothing if 16 byte FP does not
exist but we still add a comma after it and that causes a build failure.
The same is true for EXPAND_INTER_MACRO_10 too.
Committed as obvious after a bootstrap and test on x86_64-linux-gnu and aarch64-linux-gnu.
libgfortran/ChangeLog:
PR libfortran/110759
* ieee/ieee_arithmetic.F90
(COMP_INTERFACE): Remove the comma after EXPAND_INTER_MACRO_16
and EXPAND_INTER_MACRO_10.
(EXPAND_INTER_MACRO_16): Add comma here if 16 byte fp exist.
(EXPAND_INTER_MACRO_10): Likewise.