process-util: replace __sync intrinsics to __atomic

This one is rather tricky, but changing the initialization of
current_val should give this the same effect.  Based on ffmpeg's change
here: https://ffmpeg.org/pipermail/ffmpeg-devel/2014-October/164556.html

Quoting from them:
> The second reason is __atomic_compare_exchange_n(), and how it differs from
> __sync_val_compare_and_swap().
> While the latter returns *ptr as it was before the operation, the former
> doesn't and instead copies *ptr to oldval if the result of the
> comparison is false. This means that returning oldval will match the old behavoir
> without having to change the wrapper.
> A disassemble example from libavutil/buffer.o however hints that the
> __atomic function may be slower because of it writting oldval.

src/basic/process-util.c

@@ -1147,7 +1147,7 @@ void reset_cached_pid(void) {
 
 pid_t getpid_cached(void) {
         static bool installed = false;
-        pid_t current_value;
+        pid_t current_value = CACHED_PID_UNSET;
 
         /* getpid_cached() is much like getpid(), but caches the value in local memory, to avoid having to invoke a
          * system call each time. This restores glibc behaviour from before 2.24, when getpid() was unconditionally
@@ -1158,7 +1158,13 @@ pid_t getpid_cached(void) {
          * https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=c579f48edba88380635ab98cb612030e3ed8691e
          */
 
-        current_value = __sync_val_compare_and_swap(&cached_pid, CACHED_PID_UNSET, CACHED_PID_BUSY);
+        __atomic_compare_exchange_n(
+                        &cached_pid,
+                        &current_value,
+                        CACHED_PID_BUSY,
+                        false,
+                        __ATOMIC_SEQ_CST,
+                        __ATOMIC_SEQ_CST);
 
         switch (current_value) {