ppc: Introduce code deduplication macros

ppc: Introduce code deduplication macros

Introduce RSEQ_ASM_U64_PTR() which expresses the type and instance
of the defined variable, allowing deduplication of the following macros:

- __RSEQ_ASM_DEFINE_TABLE,
- RSEQ_ASM_DEFINE_EXIT_POINT.

Separate load/store/op helper macros, RSEQ_ASM_STORE_RSEQ_CS, and
RSEQ_ASM_U64_PTR into their respective ifdef regions, as the resulting
documentation is clearer (and not duplicated).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I7accb4925e1e294f2d65267641fd4c8dccb4837f