Tigthen the addition of -lgcc_eh to vxworks_libgcc_spec

Tigthen the addition of -lgcc_eh to vxworks_libgcc_spec

This change refines VXWORKS_LIBGCC_SPEC wrt the inclusion
of -lgcc_eh.

Unless the compiler features support for dual sjlj and
table based eh, libgcc_eh.a is available only with multilib
variants for which we build a shared lib (mrtp on VxWorks).

Rework logic to handle absence of libgcc_s
for -mrtp -mcmodel=large, using a conditional expr kind of
spec.

The gthread support in libgcc_eh might resort to libgcc
functions on some targets, e.g. cas synchronisation routines
on aarch64. Arrange to append -lgcc also after -lgcc_eh
in VXWORKS_LIBGCC_SPEC.

2022-10-09  Olivier Hainque  <hainque@adacore.com>

gcc/
	* config/vxworks.h (VX_LGCC_EH_SO0, VX_LGCC_EH_SO1): New
	internal macros.
	(VXWORKS_LIBGCC_SPEC): Use them and document.