add new stage 2b to dynamic linker bootstrap for thread pointer

add new stage 2b to dynamic linker bootstrap for thread pointer

commit a603a75a72bb469c6be4963ed1b55fabe675fe15 removed attribute
const from __errno_location and pthread_self, and the same reasoning
forced arch definitions of __pthread_self to use volatile asm,
significantly impacting code generation and imposing manual caching of
pointers where the impact might be noticable.

reorder the thread pointer setup and place it across a strong barrier
(symbolic function lookup) so that there is no assumed ordering
between the initialization and the accesses to the thread pointer in
stage 3.