x86: Use typeof_member style in RSEQ area access expressions

x86: Use typeof_member style in RSEQ area access expressions

Update RSEQ access macros to use `(struct rseq_area) {}.member`
in _Static_assert and __typeof expressions, instead of
RSEQ_SELF()->member.  This adopts the typeof_member style, avoiding
reliance on RSEQ_SELF for compile-time expressions.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>