Resolve host vs target num kernels issue for product et builds (#14365)

Summary:

Applying the max kernel num setting through a config/mode results in both host and target builds to limit number of kernels. This can result in a case where the host tooling is building with all ops/kernels but has the setting applied causing crashes on load. 

Host doen't  (usually) have such memory limitation as target so adding select to disable the application of max kernel num when applying a host toolchain.

Reviewed By: larryliu0820

Differential Revision: D82579896

Author: ivayloen

runtime/kernel/targets.bzl

@@ -3,7 +3,10 @@ load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "get_aten_mode_opt
 def _operator_registry_preprocessor_flags():
     max_kernel_num = native.read_config("executorch", "max_kernel_num", None)
     if max_kernel_num != None:
-        return ["-DMAX_KERNEL_NUM=" + max_kernel_num]
+        return select({
+            "DEFAULT": ["-DMAX_KERNEL_NUM=" + max_kernel_num],
+            "ovr_config//build_mode/constraints:arvr_is_host_platform": []
+        })
     elif not runtime.is_oss:
         return select({
             "DEFAULT": [],