riscv/cmake: fix Toolchain.cmake issue

This fixes the following issue when configuring canmv230:master
with Ubuntu stock toolchain and cmake 3.22:

```
CMake Error at .../arch/risc-v/src/cmake/Toolchain.cmake:218 (if):
  if given arguments:
    "STREQUAL" "GNU_RVG"
  Unknown arguments specified
```

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

arch/risc-v/src/cmake/Toolchain.cmake

@@ -215,7 +215,7 @@ if(CONFIG_DEBUG_SYMBOLS)
 endif()
 
 # Generic GNU RVG toolchain
-if(${CONFIG_RISCV_TOOLCHAIN} STREQUAL GNU_RVG)
+if(CONFIG_RISCV_TOOLCHAIN STREQUAL GNU_RVG)
 
   set(ARCHCPUEXTFLAGS i)