[UBSan] Get pc/bp for stack unwinding as early as possible.

This will ensure that stack frames in error reports will not
contain internal UBSan failures, and frame #0 will be the
actual place in the program where the error happens.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@214245 91177308-0d34-0410-b5e6-96231b3b80d8