mips: Reorganise code deduplication macros

mips: Reorganise code deduplication macros

Introduce RSEQ_ASM_U64_PTR() which expresses the type and instance
of the defined variable.

Separate the RSEQ_ASM_LONG* macros from the 64-bit pointer types, as
those two are unrelated. Having them in the same vincinity is
misleading.

Test on RSEQ_BITS_PER_LONG rather than _MIPS_SZLONG to make it
consistent across architectures.

Use both __BYTE_ORDER and __BIG_ENDIAN to detect byte order.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I1a65210e9ab0c00a5625c9feba7971081eb34c35