Upgrade ios min version to 8
Upgrade ios min version to 8

Summary:
Version 8 is the first version to support the __thread
keyword, which is required for building lsan for
64-bit ios.

Reviewers: kubamracek, kcc, glider

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D29787

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@295412 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt