Make fatal log messages be written to stderr

Make fatal log messages be written to stderr

When the loader is about to call abort(), it calls loader_log to tell the user
why the loader is crashing. Unfortunately, unless the user set VK_LOADER_DEBUG
before starting the loader, they will not see this message. This commits adds
a new debug flag to signal to loader_log() that this message should be written
to stderr no matter what. This commit also changes all current loader_log call
sites that subsequently call abort() to use this flag.