build system: Fix compilation for RISC-V with newer GCC
build system: Fix compilation for RISC-V with newer GCC

A backward incompatible change in the RISC-V resulting in instructions
previously included by rv32imac to only be available with
rv32imac_zicsr. All RISC-V CPUs supported by RIOT are hence either
considered as rv32imac (from the old ISA spec point of view) or as
rv32imac_zicsr (from the new ISA spec point of view). This adds a
simple test if GCC understands rv32imac_zicsr and uses it then as march,
but uses rv32imac as march if not.

RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kaspar030/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,miri64/RIOT,ant9000/RIOT,ant9000/RIOT,miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,kYc0o/RIOT