Make variables and functions local to translation unit (#1216)

* Make variables and functions local to translation unit

Make some global variables local to function, or remove them when
actually dead.

Signed-off-by: Marco Antognini <marco.antognini@arm.com>

* Address comments

Remove unused code.
Reduce scope of gDoubleCapabilities.

Signed-off-by: Marco Antognini <marco.antognini@arm.com>