[HWASan] Switch back to fixed shadow mapping for x86-64

For now switch back to fixed shadow mapping for x86-64 due to the issues
with IFUNC linking on older binutils. More details will be added to
https://bugs.chromium.org/p/chromium/issues/detail?id=835864

Differential Revision: https://reviews.llvm.org/D45840

llvm-svn: 330623