Infer TOOL/TOOL_FAMILY from vxworks-predef.h on VxWorks7

Infer TOOL/TOOL_FAMILY from vxworks-predef.h on VxWorks7

This change moves, for VxWorks 7, the setting of the TOOL
and TOOL_FAMILY macros from a builtin_define to a run-time
computation from vxworks-predefs.h.

This is useful on Vx7 to allow a single toolchain to be used
for instances of VxWorks based on either a gnu or an llvm system
toolchain for a given cpu (typically, powerpc).

This is achieved by leveraging the existence of a very basic
autoconf.h file in all VxWorks 7 VSBs, #included directly from
vxworks-predef.h.

gcc/
	* config/vxworks.h (VXWORKS_OS_CPP_BUILTINS): Only
	builtin_define TOOL and TOOL_FAMILY for !TARGET_VXWORKS7.
	Augment comment on VXWORKS_PERSONALITY.
	* config/vxworks/vxworks-predef.h: Infer TOOL and TOOL_FAMILY
	from the VSB autoconf.h	when we have one, determined by the presence
	of a _VSB_CONFIG_FILE definition.

libgcc/
	* config/t-vxworks: -include vxworks-predef.h explicitly, as the
	automatic inclusion is disabled by -nostdinc.