SCons: Refactor handling of production flag and per-platform LTO defaults

Fixup to #63288.
See #65583 for the bug report.

Co-authored-by: Cyberrebell <chainsaw75@web.de>

platform/linuxbsd/detect.py

@@ -171,6 +171,10 @@ def configure(env):
             env.Append(LINKFLAGS=["-fsanitize=memory"])
 
     # LTO
+
+    if env["lto"] == "auto":  # Full LTO for production.
+        env["lto"] = "full"
+
     if env["lto"] != "none":
         if env["lto"] == "thin":
             if not env["use_llvm"]: