Add missing extensions to Kelvin's march setting

- Add zicsr, zifencei, zbb, and set ISA versions for I.

Change-Id: I647f2ded48e82e6ed796b07667597626350af501

cmake/riscv_iree_gcc.cmake

diff --git a/cmake/riscv_iree_gcc.cmake b/cmake/riscv_iree_gcc.cmake
index fdc9b6e..0f59f1c 100644
--- a/cmake/riscv_iree_gcc.cmake
+++ b/cmake/riscv_iree_gcc.cmake
@@ -57,7 +57,7 @@
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_C_EXTENSIONS OFF)     # Force the usage of _ISOC11_SOURCE
 set(CMAKE_SYSTEM_LIBRARY_PATH "${RISCV_TOOLCHAIN_ROOT}/riscv32-unknown-elf/lib")
-set(RISCV_COMPILER_FLAGS "${RISCV_COMPILER_FLAGS} -march=rv32im -mabi=ilp32")
+set(RISCV_COMPILER_FLAGS "${RISCV_COMPILER_FLAGS} -march=rv32i2p1m_zicsr_zifencei_zbb -mabi=ilp32")
 
 add_compile_definitions(IREE_PLATFORM_GENERIC)