freebsd.h (LIB_SPEC): Key the exact setting to configure-time options and target version of FreeBSD.

	* config/freebsd.h (LIB_SPEC): Key the exact setting to
	configure-time options and target version of FreeBSD.  Set-up
	to match architectural change in threading support.  Correctly
	report when -pthread option is not supported by configure-time
	options.
	(FBSD_CPP_PREDEFINES): Move from architecture-specific files
	and extend to encode target version number in run-time setting
	of __FreeBSD__ macro to match the system compiler.
	(CPP_PREDEFINES): Likewise.
	(PREFERRED_DEBUGGING_TYPE): Change to DWARF2_DEBUG.

	* config/alpha/freebsd.h (CPP_PREDEFINES): Moved to common freebsd.h.
	* config/i386/freebsd.h (CPP_PREDEFINES): Likewise.

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

From-SVN: r42464