[PATCH] Unify pthread_spin_[try]lock implementations.

[Roland McGrath]

> It seems to me that SPIN_LOCK_READS_BETWEEN_CMPXCHG may have the 
> property that varying it at runtime based on the characteristics of the 
> particular chip running the application might be worthwhile.

Nothing prevents a machine from defining the macro to use a global variable
that is set by some machine-specific initialization code.