[HWASan] Fixed slow DWARF unwinding.

Summary: CFA was setup incorrectly, as there is an 8-byte gap at the top of the stack for SP 16-byte alignment purposes.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: kubamracek, javed.absar, #sanitizers, llvm-commits, pcc

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D60798

llvm-svn: 358535