Atomics: several fixes the GCC backend

[devreal]

• GCC's acquire thread-fence was busted until GCC 8.1.0 (similar for C11). Check godbolt for an example compiled with GCC 7 and 8. Notice that the load of x is optimized out of the loop when compiling with GCC 7. We had previously updated the version of the busted GCC for the C11 atomics (opal_atomic_rmb: GCC has fixed its acquire fence barrier in its 8 release #10118).
• Fix initialization of locks (if HLE is enabled)

Signed-off-by: Joseph Schuchart [email protected]