arm64: app_setting: Fix race on the count variable

Currently `count' is being incremented with a postfix ++ in the same
statement as the assignment of the array element that the old count
value indexes.  This is a problem for two reasons:

  (1) C doesn't guarantee that the count happens after the
      store. Postfix ++ yields a copy of the old value of the variable
      before it was incremented.  The actual incrementing of the
      variable could happen at any time before the end of the
      statement (possibly before the assignment).

  (2) Even if we fix (1) by pulling the increment out, ARM doesn't
      guarantee that the instructions won't be re-ordered.  The
      instructions could be re-ordered such that the count update
      happens before the store.

Therefore, there could be a race where count has already been
incremented but the new array element hasn't yet been updated.  Fix this
by pulling the increment out of the array index, and placing a memory
barrier between the array update and the increment.

CRs-Fixed: 997757
Change-Id: Iaccba6b5bb33ce8fa720811651692a6114deaf3a
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>