add --enable-rtld-now in upgrade info

Author: remicollet

UPGRADING.INTERNALS

@@ -200,6 +200,8 @@ PHP 7.4 INTERNALS UPGRADE NOTES
       since they are not needed on the current systems.
     - Local PHP macros PHP_TARGET_RDYNAMIC, PHP_SOLARIS_PIC_WEIRDNESS,
       PHP_SYS_LFS have been removed.
+    - new --enable-rtld-now build option allow to switch dlopen behavior
+      from RTLD_LAZY to RTLD_NOW
 
   c. Windows build system changes