Packages that use 64-bit atomics carry open-coded architecture lists
such as +(arm||armeb||mips||mipsel||powerpc):libatomic. Those lists
drift (ruby still names arc), over-include ARMv6K and ARMv7 targets
whose compiler inlines 64-bit atomics via ldrexd/strexd, and
under-include ARMv5 in other packages. Add HAS_ATOMIC64, selected on
the architectures where GCC emits 64-bit atomics inline (all 64-bit
ones, x86 via cmpxchg8b, ARMv6K and ARMv7), and ATOMIC64_DEPENDS as
the single dependency expression to use instead. The condition is one
negated symbol because the Depends: evaluator in package-pack.mk only
handles flat conjunctions and disjunctions. 16-byte and size-generic
atomics need libatomic on every architecture and keep a plain
+libatomic dependency.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>