Setup TARGET_C_PREINCLUDE for VxWorks

Setup TARGET_C_PREINCLUDE for VxWorks

This patch modifies the C & C++ VxWorks compiler to predefine
the __STDC_ macros verified by gcc.dg/c99-predef-1.c in the testsuite.

2019-12-13  Joel Brobecker  <brobecker@adacore.com>

	* config.gcc <*-*-vxworks*>: Add vxworks-c.o to c_target_objs
	and cxx_target_objs. Set target_has_targetcm to "yes". Add
	vxworks-predef.h to extra_headers.
	* config/t-vxworks (vxworks-c.o): New target.
	* config/vxworks-c.c: New file.
	* config/vxworks/vxworks-predef.h: New file.

From-SVN: r279781