Update RISCV CMake build default configurations

* Avoid duplicated compile flags
* Set the default optimization level to -O3 (aligned with
  CMAKE_BUILD_TYPE=Release)
* Fix RV64 cmake config
diff --git a/build_tools/cmake/build_riscv.sh b/build_tools/cmake/build_riscv.sh
index f1f0c1e..641a44c 100755
--- a/build_tools/cmake/build_riscv.sh
+++ b/build_tools/cmake/build_riscv.sh
@@ -19,6 +19,7 @@
 ROOT_DIR="${ROOT_DIR:-$(git rev-parse --show-toplevel)}"
 CMAKE_BIN="${CMAKE_BIN:-$(which cmake)}"
 RISCV_CONFIG="${RISCV_CONFIG:-rv64}"
+RISCV_COMPILER_FLAGS="${RISCV_COMPILER_FLAGS:--O3}"
 
 "${CMAKE_BIN?}" --version
 ninja --version
@@ -67,6 +68,7 @@
   -DCMAKE_TOOLCHAIN_FILE="$(realpath ${ROOT_DIR?}/build_tools/cmake/riscv.toolchain.cmake)"
   -DIREE_HOST_BINARY_ROOT="$(realpath ${BUILD_HOST_DIR?}/install)"
   -DRISCV_CPU="${RISCV_CONFIG?}"
+  -DRISCV_COMPILER_FLAGS="${RISCV_COMPILER_FLAGS?}"
   -DIREE_ENABLE_ASSERTIONS=ON
   -DIREE_BUILD_COMPILER=OFF
   -DIREE_BUILD_SAMPLES=ON