Introduce internal_sysctlbyname in place of sysctlbyname

Summary:
This change will allow to install sysctlbyname() interceptors
more easily in sanitizers.

Reviewers: vitalybuka, joerg

Reviewed By: vitalybuka

Subscribers: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D52793

llvm-svn: 343840