Update compiler architecture flag to work with modern GCC.

Author: disdi

sw/c/gcc_toolchain.cmake

@@ -3,6 +3,6 @@ set(CMAKE_SYSTEM_NAME Generic)
 set(CMAKE_C_COMPILER riscv32-unknown-elf-gcc)
 set(CMAKE_OBJCOPY riscv32-unknown-elf-objcopy)
 set(CMAKE_C_FLAGS_INIT
-    "-march=rv32imc -mabi=ilp32 -mcmodel=medany -Wall -fvisibility=hidden -ffreestanding")
+    "-march=rv32ima_zicsr_zifencei -mabi=ilp32 -mcmodel=medany -Wall -fvisibility=hidden -ffreestanding")
 set(CMAKE_ASM_FLAGS_INIT "-march=rv32imc")
 set(CMAKE_EXE_LINKER_FLAGS_INIT "-nostartfiles -T \"${LINKER_SCRIPT}\"")