soc: openisa_rv32m1: Update target architecture for GCC 12

This commit updates the custom target architecture type specified for
the `SOC_OPENISA_RV32M1_RI5CY` and `SOC_OPENISA_RV32M1_ZERO_RISCY` SoC
types to be compatible with the GCC 12, which now uses the ISA spec
20191213 by default.

Note that the hack overriding the build system-default `-march` flag
for these SoCs needs to be removed and they should be properly
specified using the ISA extension Kconfigs.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>