Guard additional libm functions with SKIP_C_FUNC_REDEF

I'm looking to include core-math into an external application as a shared library. In order to make sure that all of the math functions go through core-math and not libm, I have redefined all of the cr_xxx functions as their libm counterparts. When doing so however, I ran into duplicate function definition errors due to these redefinitions inside core-math. By guarding them with SKIP_C_FUNC_REDEF, the errors are resolved.