Set default visiblity of functions to hidden

Set default visiblity of functions to hidden

Reduce the number of functions that are exported by the layer. This
helps resolve dynamic linking issues where multiple functions are named
the same in different shared libraries. Also add function specifiers and
calling conventions to all extern "C" functions.

Change-Id: I07b33ff8d066e33c5dbdf0cbc13aa7835a78220b
Signed-off-by: Dennis Tsiang <dennis.tsiang@arm.com>