bionic: add missing memory barriers to system properties

bionic: add missing memory barriers to system properties

1) Reading the value must finish before checking whether it's intact

2) Setting the serial's dirty bit must visible before modifying the
value

3) The modified value must be visible before clearing the serial's dirty
bit

4) New properties and their TOC entries must be visible before updating
the property count

Signed-off-by: Greg Hackmann <ghackmann@google.com>
(cherry picked from commit 5bfa3ee8b37ef162154559575193018a6235acba)

Change-Id: Id3fa45261fc2df2ae493ab5194bc2b6bff04e966