BACKPORT: arm64: barriers: introduce nops and __nops macros for NOP sequences

NOP sequences tend to get used for padding out alternative sections
and uarch-specific pipeline flushes in errata workarounds.

This patch adds macros for generating these sequences as both inline
asm blocks, but also as strings suitable for embedding in other asm
blocks directly.

Signed-off-by: Will Deacon <will.deacon@arm.com>

Bug: 31432001
Change-Id: I7f82b677a065ede302a763d39ffcc3fef83f8fbe
(cherry picked from commit f99a250cb6a3b301b101b4c0f5fcb80593bba6dc)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>