x86: Fix THREAD_SELF definition to avoid ld.so crash (bug 27004)
The previous definition of THREAD_SELF did not tell the compiler that %fs (or %gs) usage is invalid for the !DL_LOOKUP_GSCOPE_LOCK case in _dl_lookup_symbol_x. As a result, ld.so could try to use the TCB before it was initialized. As the comment in tls.h explains, asm volatile is undesirable here. Using the __seg_fs (or __seg_gs) namespace does not interfere with optimization, and expresses that THREAD_SELF is potentially trapping.
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