[parisc-linux] ldcw in __pthread_acquire

[Alan Cox]

> As Lamont Jones agreed, lightweight system calls exist, and their
> cost is on the order of a couple of instructions (typically the equivalent
> of a missed branch prediction).  HP-UX has some, Lamont speaks from experience
> (as do I).

x86 is about 50 clocks to do a syscall so the maths is strongly in favour
of user mode spins.

> Thus, it is *NOT* true that a syscall cost must be "huge".

On 99.99% of the machines people care about it is 8)