Arrange to preinclude yvals.h ahead of stdint on VxWorks 7

Arrange to preinclude yvals.h ahead of stdint on VxWorks 7

On Vxworks 7, includers of stdint.h (which we currently "provide")
need yvals.h to have been included ahead.

Instead of altering the common stdint-gcc.h with unpleasant
vxworks specific bits to do that, we arrange to provide stdint-gcc.h
on its own along with a stdint.h wrapper which preincludes yvals.h
on vx7 then includes stdint-gcc.h.

2019-12-30  Olivier Hainque  <hainque@adacore.com>

	* config/vxworks/stdint.h: New file.  Include _yvals.h
	then stdint-gcc.h.
	* config/t-vxworks: Arrange to install the stdint.h wrapper.
	* config.gcc (*-*-vxworks*): Add stdint-gcc.h to $extra_headers
	so it gets copied.  Set use_gcc_stdint to request _not_ crafting
	stdint.h through the common Makefile rules.

From-SVN: r279791