[hwasan,asan] Intercept vfork.

Summary: AArch64 only for now.

Reviewers: vitalybuka, pcc

Subscribers: srhines, kubamracek, mgorny, javed.absar, krytarowski, kristof.beyls, jdoerfert, #sanitizers, llvm-commits, kcc

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D58313

llvm-svn: 354625