How to tell gcc to stop using built-in functions?

I am using my own modified glibc. I saw in the compiled code that compiler was not using many standard library functions from my glibc when I linked with it. Then I put -fno-builtin flag. Things got better and I could see that many functions which were not taken from glibc were now taken from there, such as malloc.

However, still for many functions, such as mmap, the compiler is using some built-in-code. Now how can I ask the compiler to please exclusively use the code from glibc rather than using its built-in-functions?

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