Add upcoming_changes note

doc/release/upcoming_changes/21154.improvement.rst

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+Math C library feature detection now works for wasm
+---------------------------------------------------
+The previous feature detection generated C stubs like: 
+
+.. code-block:: C
+
+    void exp(void); // incorrect signature!
+    exp();
+    
+Then checked whether compiling and linking these worked. This generates warnings
+that complain that ``exp`` actually has signature ``double exp(double)`` but would
+otherwise work. However ``wasm-ld`` creates a hard error in this case, so when
+compiling to a wasm target everything is detected as missing.
+
+This is now fixed: we feature detect instead with code like:
+
+.. code-block:: C
+
+    double exp(double); // correct signature!
+    exp((double)0);
+
+which ``wasm-ld`` will link. As a side effect, the feature detection code now
+works without generating linker warnings.