fix rv64gcv test coverage build

toolchains/riscv64-unknown-linux-gnu.toolchain.cmake

@@ -18,8 +18,8 @@ set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
 set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
 set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
 
-set(CMAKE_C_FLAGS "-march=rv64gc")
-set(CMAKE_CXX_FLAGS "-march=rv64gc")
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -march=rv64gc")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -march=rv64gc")
 
 # cache flags
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}" CACHE STRING "c flags")