Add 'global' driver option to erl_ddll:try_load/3

On systems using dlopen() to load a driver, it may be necessary
to use the RTLD_GLOBAL flag.

This patch adds support for the 'global' atom in 'driver_options'
eg: {driver_options, [global]}

Parts of erts using dlopen() unrelated to try_load/3 have been
set to use the RTLD_NOW flag, as it is the current behaviour
Merge branch 'erl_ddll_try_load0' of https://github.com/oliv3/otp into erl_ddll_try_load0