Introduce vxworks7r2 support for ppc and ppc64

This change introduces support for the most recent versions
of VxWorks on PowerPC targets, for both 32 and 64 bit thanks
to a bi-arch setup.

The system compilers are essentially configured as Linux
toolchains with only a few specificities and we replicate
that model here.

The most visible specificities are the use of secureplt by
default, the pre-definition of some macros that the system
headers still rely on (_VX_CPU and _VX_CPU_FAMILY, for example),
and of course some variations related to the so VxWorks typical
kernel vs RTP mode distinction.

In addition to the introduction of config.gcc and libgcc
configuration chunks, much inspired by the linux ones, the
change

- Reworks rs6000/vxworks.h file to feature bits common to the
  Vx6 and Vx7 port then a separate section for each, where the
  Vx7 part is very short as we rely on the Linux definitions
  for most things.

- Adjusts the CPU macro predefinitions in CPP_SPEC to resort
  to "_VX_CPU" instead of "CPU" for Vx7, to better match the
  more recent system headers expectations,

- Adds a cpu definition case for e6500.

- Changes to the use SUB3TARGET_OVERRIDE_OPTIONS instead of
  SUBSUBTARGET_OVERRIDE_OPTIONS for specifics, so we don't
  override the Linux's version of the latter for vx7.

2020-10-20  Douglas Rupp  <rupp@adacore.com>

gcc/
	* config.gcc (powerpc*-wrs-vxworks7r*): New case.
	* config/rs6000/vxworks.h: Rework to handle VxWorks7.
	Refactor as common bits + vx6 vs vx7 ones. For the
	latter, rely essentially on the Linux configuration
	and adjust CPU to _VX_CPU in CPP_SPEC. Add a case
	for e6500. Use SUB3TARGET_OVERRIDE_OPTIONS for specifics
	to preserve the Linux SUBSUBTARGET_OVERRIDE_OPTIONS
	for vx7.

libgcc/
	* config.host (powerpc*-wrs-vxworks7*): New case.
	* configure.ac: Handle powerpc*-*-vxworks7* as
	powerpc*-*-linux* for ppc-fp_type.
	* configure: Regenerate.

Co-authored-by: Olivier Hainque <hainque@adacore.com>