gmodule: Use RTLD_DEFAULT if defined __BIONIC__

This is a partial change of the previous work[0].
On 64 bit Android since android-23, 'handle = dlopen(NULL); dlsym(handle)'
doesn't work. Instead, only 'dlsym(RTLD_DEFAULT)' returns a valid pointer.

However, RTLD_DEFAULT is defined as '(void *) 0x0' on 64bit Android which
is usually used for invalid value so this patch allows the specific case.

[0] 0d81bb4e

https://bugzilla.gnome.org/show_bug.cgi?id=788270