Arm backend: Change run.sh to let cmake decide number of parallel jobs (pytorch#5813)

Summary:
Instead of specifying number of parallel jobs let cmake decide for it self. This make this script work better on macOS and might speed up builds as more parallel jobs are used.

Pull Request resolved: pytorch#5813

Reviewed By: mergennachin

Differential Revision: D63809604

Pulled By: digantdesai

fbshipit-source-id: 905fde94a270cd2cacb1257857962ad507a4cbc5

Author: zingo

examples/arm/run.sh

@@ -119,8 +119,7 @@ function build_quantization_aot_lib()
         -Bcmake-out-aot-lib \
         "${et_root_dir}"
 
-    n=$(nproc)
-    cmake --build cmake-out-aot-lib -j"$((n - 5))" -- quantized_ops_aot_lib
+    cmake --build cmake-out-aot-lib --parallel -- quantized_ops_aot_lib
 }
 
 
@@ -148,8 +147,7 @@ function build_executorch() {
 
     echo "[${FUNCNAME[0]}] Configured CMAKE"
 
-    n=$(nproc)
-    cmake --build ${et_build_dir} -j"$((n - 5))" --target install --config Release
+    cmake --build ${et_build_dir} --parallel --target install --config Release
 
     cmake                                                 \
         -DCMAKE_INSTALL_PREFIX=${et_build_dir}            \
@@ -159,7 +157,7 @@ function build_executorch() {
         -DCMAKE_TOOLCHAIN_FILE="${toolchain_cmake}"       \
         -B"${et_build_dir}"/examples/arm                  \
         "${et_root_dir}"/examples/arm
-    cmake --build ${et_build_dir}/examples/arm -- -j"$((n - 5))"
+    cmake --build ${et_build_dir}/examples/arm --parallel --
 
     set +x
 
@@ -190,8 +188,7 @@ function build_executorch_runner() {
 	  -DPYTHON_EXECUTABLE=$(which python3)
     echo "[${FUNCNAME[0]}] Configured CMAKE"
 
-    n=$(nproc)
-    cmake --build ${executor_runner_path}/cmake-out -- -j"$((n - 5))" arm_executor_runner
+    cmake --build ${executor_runner_path}/cmake-out --parallel -- arm_executor_runner
     echo "[${FUNCNAME[0]}] Generated baremetal elf file:"
     find ${executor_runner_path}/cmake-out -name "arm_executor_runner"
     echo "executable_text: $(find ${executor_runner_path}/cmake-out -name arm_executor_runner -exec size {} \; | grep -v filename | awk '{print $1}') bytes"