Add opt-in LTO for portable executor (#24001)

Summary:

Add a generic, default-off `executorch.enable_lto` Buck configuration for the portable `program_no_prim_ops` target. When enabled, the target compiles with `-flto=thin` and exports `-flto=thin` to Buck-linked consumers. OSS builds and ATen variants remain unchanged.

Thin mode is explicit so a target-local opt-in cannot override a product's ThinLTO pipeline with full LTO. Orca already has independently landed whole-XRA/Kymera ThinLTO from D125548482; enabling this setting there is mode-consistent and is not credited with product-wide LTO savings.

Reviewed By: JacobSzwejbka

Differential Revision: D125783739

Author: ivayloen

runtime/executor/targets.bzl

@@ -28,6 +28,14 @@ def _program_preprocessor_flags():
         fail("executorch.enable_program_verification must be one of 'true' or 'false'; saw '" +
              enable_verification + "'")
 
+def _program_lto_flags(aten_mode):
+    if runtime.is_oss or aten_mode:
+        return []
+    return select({
+        "DEFAULT": [],
+        "fbsource//xplat/executorch/tools/buck/constraints:executorch-lto-enabled": ["-flto=thin"],
+    })
+
 def define_common_targets():
     """Defines targets that should be shared between fbcode and xplat.
 
@@ -107,7 +115,8 @@ def define_common_targets():
             compiler_flags = select({
                 "ovr_config//os:windows": [],
                 "DEFAULT" :["-Wno-error=deprecated-declarations"]
-            }),
+            }) + _program_lto_flags(aten_mode),
+            exported_linker_flags = _program_lto_flags(aten_mode),
             preprocessor_flags = _program_preprocessor_flags(),
             exported_deps = [
                 ":memory_manager",

tools/buck/constraints/BUCK

@@ -75,6 +75,14 @@ fb_native.constraint_value(
     visibility = ["PUBLIC"],
 )
 
+fb_native.config_setting(
+    name = "executorch-lto-enabled",
+    values = {
+        "executorch.enable_lto": "true",
+    },
+    visibility = ["PUBLIC"],
+)
+
 fb_native.config_setting(
     name = "executorch-program-verification-disabled",
     constraint_values = [