compiler-rt: use __GLIBC_PREREQ for SANITIZER_INTERCEPT_GETRANDOM

Summary: Fixes https://github.com/google/oss-fuzz/issues/2836

Reviewers: eugenis

Subscribers: dberris, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D68178

llvm-svn: 373284