config.gcc (the unlabeled system switch statement): Add generic configuration section to handle *-*-freebsd*.

	* config.gcc (the unlabeled system switch statement): Add
	generic configuration section to handle *-*-freebsd*.  Also,
	to match the system compiler, default to threading support (it
	may be disabled with the standard --disable-threads
	configuration-time switch).
	(i[34567]86-*-freebsd* in machine-specific section): Use it.
	(alpha*-*-freebsd* in machine-specific section): Likewise.

	* config/freebsd3.h: New file to expose FBSD_MAJOR macro (3).
	* config/freebsd4.h: New file to expose FBSD_MAJOR macro (4).
	* config/freebsd5.h: New file to expose FBSD_MAJOR macro (5).
	* config/freebsd6.h: New file to expose FBSD_MAJOR macro (6).
	* config/freebsd-nthr.h: New file to expose FBSD_NO_THREADS macro.

Co-Authored-By: David O'Brien <obrien@freebsd.org>

From-SVN: r42463