Improve comments and INITFINI macro use in vxcrtsutff.c

This change augments the comment attached to the use of auto-host.h
in vxcrtstuff.c to better describe the reason for including it and
for the associated series of #undef directives.

It also augments the comment on dso_handle and removes a redundant
guard on HAVE_INITFINI_ARRAY_SUPPORT for the shared version of the
objects, nested within a section guarded on USE_INITFINI_ARRAY.

2022-09-29  Olivier Hainque  <hainque@adacore.com>

libgcc/
	* config/vxcrtstuff.c: Improve the comment attached to the use
	of auto-host.h and of __dso_handle.  Remove redundant guard on
	HAVE_INITFINI_ARRAY_SUPPORT within a USE_INITFINI_ARRAY section.