x86-64: use %fs segment selector for accessing rseq thread area

x86-64: use %fs segment selector for accessing rseq thread area

Rather than use rseq_get_abi() and pass its result through a register to
the inline assembler, directly access the per-thread rseq area through a
memory reference combining the %fs segment selector, the constant offset
of the field in struct rseq, and the rseq_offset value (in a register).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ia0c93d32b9b36ee1ef6a737a8a5321498fc8c3bc