Use Unikraft initcall for initialization

Use Unikraft initcall for initialization

Initialize the library by using an Unikraft initcall instead of the
constructor attribute. This is to make sure that pthread-embedded is
initialized before Unikraft calls any application constructors and
after Unikraft initialized the depending scheduler.

Signed-off-by: Simon Kuenzer <simon.kuenzer@neclab.eu>
Reviewed-by: Sharan Santhanam <sharan.santhanam@neclab.eu>