re PR other/7956 (unreadable error on -m64 with --disable-multilib)

re PR other/7956 (unreadable error on -m64 with --disable-multilib)

	PR other/7956
	* genmultilib: New variable disable_multilib.  Set it to 'yes'
	if enable_multilib was set to 'no'.  Emit DISABLE_MULTILIB
	if disable_multilib was set to 'yes'.
	* gcc.c: Include multilib.h before tm.h.
	* config/sparc/sol2-bi.h (LINK_ARCH_SPEC): Emit an error
	message for multiarch options if DISABLE_MULTILIB is set.
	* config/sparc/sol2-gld-bi.h (LINK_ARCH_SPEC): Likewise.

From-SVN: r74875