Ada: add missing support for the S/390 and RISC-V architectures

...to the object file reader present in the run-time library.

gcc/ada/
	PR ada/118459
	* libgnat/s-objrea.ads (Object_Arch): Add S390 and RISCV.
	* libgnat/s-objrea.adb (EM_S390): New named number.
	(EM_RISCV): Likewise.
	(ELF_Ops.Initialize): Deal with EM_S390 and EM_RISCV.
	(Read_Address): Deal with S390 and RISCV.