libstdc++: Fix @file for target-specific opt_random.h

A few of these files self-identified as ext/random.tcc, update to use
the actual basename.

libstdc++-v3/ChangeLog:

	* config/cpu/aarch64/opt/ext/opt_random.h: Improve doxygen file
	docs.
	* config/cpu/i486/opt/ext/opt_random.h: Likewise.