soc/openisa: rely on the `CONFIG_RISCV_ISA_EXT_*` options for arch string

This commit makes the RV32M1 SoC rely on the default behavior of relying on
the `CONFIG_RISCV_ISA_EXT_*` config options, and removes the
`zephyr_compile_options` override when the standard toolchain is used.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>