Resolve __va_list_tag ** -> va_list * type errors with clang and future GCCs

Functions taking va_args are munged to receive arguments of type
'__va_list_tag *'. This patch uses va_copy to coerce them to the expected type
so we don't get compiler errors.

Tested on x86_64, both 32-bit and 64-bit compiles.

Reported-By: Basile Starynkevitch <basile@starynkevitch.net>