ARM64: Avoid LD3/ST3 at run time, not compile time

ARM64: Avoid LD3/ST3 at run time, not compile time

... and only if ThunderX is detected.  This can be easily expanded later
on to include other CPUs that are known to suffer from slow LD3/ST3, but
it doesn't make sense to disable LD3/ST3 for all non-Android Linux
platforms just because ThunderX is slow.