[KCFI] Fix hash offset calculation in Thumb mode

ARM stores the Thumb state in the least significant bit of the
function pointers. When compiling for ARM or Thumb, as all
instructions are at least 16-bit aligned, ignore the LSB when
computing the prefix hash location, so we can support both
pure Thumb and mixed ARM/Thumb binaries.

Fixes #62936

Reviewed By: MaskRay, simon_tatham

Differential Revision: https://reviews.llvm.org/D152484