Xcode: Suppress default Objective C/C++ flag -fobjc-link-runtime

Xcode projects add this link flag by default for Objective C/C++.  That
is not consistent with the Makefile and Ninja generator behavior.  Tell
Xcode not to add this flag.  Project code can add it if needed and it
will then be used for all generators.

Fixes: #19990
Status Job ID Name Coverage
  External
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