MINOR: threads/build: atomic: replace the few inlines with macros

MINOR: threads/build: atomic: replace the few inlines with macros

[ plock commit 61e255286ae32e83e1a3174dd7c49eda99880a8b]

There are a few inlines such as pl_barrier() and pl_cpu_relax() which
are used a lot. Unfortunately, while building test code at -O0, inlining
is disabled and these ones are called a lot and show up a lot in any
profile, are traced into when single-stepping with a debugger, etc, thus
they are polluting the landscape. Since they're single-asm statements,
there is no reason for not turning them into macros.

The result becomes fairly visible here at -O0 :

  $ size latency.inline latency.macro
     text    data     bss     dec     hex filename
    11431     692     656   12779    31eb treelock.inline
    10967     692     656   12315    301b treelock.macro

And it was verified that regularly optimized code remains strictly identical.