[sanitizer] Use _thread_db_sizeof_pthread to obtain struct pthread size

This symbol has been exported (as an internal GLIBC_PRIVATE symbol) from libc.so.6 starting with glibc 2.34. glibc uses it internally for its libthread_db implementation to enable thread debugging on GDB, so it is unlikely to go away for now.

Fixes #52989.

Reviewed By: #sanitizers, MaskRay, vitalybuka

Differential Revision: https://reviews.llvm.org/D119007