Fix PR 19089: Environment variable TMP may yield gcc: abort

Even though I cannot reproduce the ICE any more, this is still
a bug. We check already to see if we can access the directory
but never check to see if the path is actually a directory.

This adds the check and now we reject the file as not usable
as a tmp directory.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

libiberty/ChangeLog:

	* make-temp-file.c (try_dir): Check to see if the dir
	is actually a directory.