Some improvements CMAKE_OSX_ARCHITECTURES / codec class subclassing / AP4_FragmentSampleTable

[CastagnaIT]

i choosen propose to send some less invasive changes from our custom bento4 in the hoping will be reviewed cc @barbibulle

commit 1:
CMAKE_OSX_ARCHITECTURES has been hardcoded, this lead to problems when you dont need to change it, at least on our build system dont need it, and this cause build failures.

my solution proposal is allow use of new bool: USE_DEFAULT_OSX_ARCHITECTURES
to allow to avoid set the default values
example: -DUSE_DEFAULT_OSX_ARCHITECTURES=OFF
the previous behaviour is kept without changes, but allow to disable it
fix #945

commit 2:
as title implement GetSampleIndexForTimeStamp/GetNearestSyncSampleIndex methods

commit 3:
allow codec audio parsers classes to be inherited, and that subclasses can access to private data/methods