[LLDB/Coredump] Only take the Pthread from stack start to the stackpointer + red_zone (#92002)

Currently in Core dumps, the entire pthread is copied, including the
unused space beyond the stack pointer. This causes large amounts of core
dump inflation when the number of threads is high, but the stack usage
is low. Such as when an application is using a thread pool.

This change will optimize for these situations in addition to generally
improving the core dump performance for all of lldb.