On AArch64, use clone() instead of fork()

Patch by Gideon Billings.

Differential Revision: http://reviews.llvm.org/D3903

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@209641 91177308-0d34-0410-b5e6-96231b3b80d8