Introduce support for vxworks7r2 on x86 and x86_64

Introduce support for vxworks7r2 on x86 and x86_64

This change extends the VxWorks support on intel CPUs to
VxWorks7r2 for x86_64 as well as x86, with a "mcmodel=large"
additional multilib for the 64bit configuration.

The support for fPIC is not functional yet for this model,
so we just don't add the corresponding multilib.

We extend the range of CPU families handled by TARGET_OS_CPP_BUILTINS,
accounting for the fact that archs older than PENTIUM4 are
not supported (any more) by VxWorks 7.

As we did for powerpc, we leverage VX_CPU_PREFIX to emit different
forms of definitions for different families of VxWorks as the system
headers's expectations has evolved between Vx 5, 6 and 7.

2020-10-27  Olivier Hainque  <hainque@adacore.com>

gcc/
	* config.gcc: Adjust the ix86/x86_64-wrs-vxworks filters
	to apply to VxWorks 7 as well.
	* config/i386/t-vxworks (MULTILIB_OPTIONS, MULTILIB_DIRNAMES):
	Remove the fPIC multilib and add one for the large code model
	on x86_64.
	* config/i386/vxworks.h: Separate sections for TARGET_VXWORKS7,
	other variants and common bits.
	(TARGET_OS_CPP_BUILTINS): Augment to support a range of CPU
	families. Leverage VX_CPU_PREFIX.
	(CC1_SPEC): Add definition.
	(STACK_CHECK_PROTECT): Use conditional expression instead of
	heavier to read conditioned macro definitions.

libgcc/
	* config.host: Adjust the ix86/x86_64-wrs-vxworks filters
	to apply to VxWorks 7 as well.

Co-authored-by: Douglas Rupp <rupp@adacore.com>
Co-authored-by: Pat Bernardi <bernardi@adacore.com>