Fix some dangling references to `netbsd-tdep`

Fix some dangling references to `netbsd-tdep`

These files were renamed in 1b71cfcfdc3e13a655fefa6566b5564cec044c10,
but evidentially a few dangling references were left behind. This causes
builds to fail:

    $ ./configure --target i686-netbsdelf
    $ make
    make: *** No rule to make target 'nbsd-tdep.c', needed by 'nbsd-tdep.o'.  Stop.

gdb/ChangeLog:

	* sparc-tdep.h: Fix comment.
	* netbsd-tdep.c (nbsd_info_proc_mappings_header): Fix comment.
	(nbsd_init_abi): Fix comment.
	* configure.tgt (*-*-netbsd* | *-*-knetbsd*-gnu): Fix netbsd
	file name.
	(alpha*-*-openbsd*): Likewise.
	(sparc-*-openbsd*): Likewise.
	(sparc64-*-openbsd*): Likewise.

Change-Id: I18a0873902dccadd238615577aac4e08772fa2c8