Rework VXWORKS_LINK_SPEC for shared objects support

Rework VXWORKS_LINK_SPEC for shared objects support

Split LINK_SPEC as BASE_LINK_SPEC + EXTRA_LINK_SPEC,
with an overridable LINK_OS component that cpu ports may
redefine.

Leverage the latter on powerpc for VxWorks 7, where we incorporate
our specific bits in the linux os configuration as the system compiler
is now very close to a standard linux one.

The split allows supporting shared objects (shared libs and
non-static rtps) on recent versions of VxWorks while retaining
compatibility with older VxWorks targets which could link with
shared libraries but not build them.

2021-12-07  Doug Rupp  <rupp@adacore.com>
	    Olivier Hainque  <hainque@adacore.com>

gcc/
	* config/vxworks.h (VXWORKS_LINK_OS_SPEC): New spec.
	(VXWORKS_BASE_LINK_SPEC): New spec, using the former.
	(VXWORKS_EXTRA_LINK_SPEC): New spec for old and new VxWorks.
	(VXWORKS_LINK_SPEC): Combo of BASE and EXTRA specs.
	* config/rs6000/vxworks.h (VXWORKS_LINK_OS_SPEC): Empty.
	(LINK_OS_EXTRA_SPEC32): Use VXWORKS_LINK_SPEC.
	(LINK_OS_EXTRA_SPEC64): Likewise.