[sanitizer] Correct GetTls for x32

Since x32 pointer size is 4 bytes, the self pointer offset in TCB:

struct
{
  void *tcb;
  dtv_t *dtv;
  void *self;
  ...

should be 8, not 16.

Fixes https://github.com/llvm/llvm-project/issues/55288

Differential Revision: https://reviews.llvm.org/D125025