Replace t-ppccomm by t-vxworks in libgcc/config/rs6000

Replace t-ppccomm by t-vxworks in libgcc/config/rs6000

This removes ibm-ldouble.c and a few eabi crt files from the build
closure, which were producing objects we don't use anyway.

2021-12-07  Rasmus Villemoes  <rv@rasmusvillemoes.dk>

libgcc/
	* config/rs6000/t-vxworks: New file.
	* config.host (powerpc*-*-vxworks*): Use it instead of
	t-ppccomm.