tsan: fix out-of-bounds access in Go runtime
FuncEntry can resize the shadow stack, while "thr->shadow_stack_pos[0] = pc" writes out-of-bounds.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@210349 91177308-0d34-0410-b5e6-96231b3b80d8