Add LTO back to non-Windows builds

The Windows case (the option name is "/GL") is more complicated, since
there, the compiler options also need to be passed to the linker if LTO
is enabled.

Since we are gating the linker options on platform at the moment instead
of compiler, we need to implement a Bazel boolean flag for the case
"Platform == MacOS && Compiler == AnyOf(gcc, clang)".

bindings/python/nanobind.BUILD

@@ -21,8 +21,10 @@ cc_library(
             "/EHsc",  # exceptions
             "/Os",  # size optimizations
         ],
+        # these should work on both clang and gcc.
         "//conditions:default": [
             "-fexceptions",
+            "-flto",
             "-Os",
         ],
     }),