Remove ppc*-vxworks7* inadequate libgcc Makefile fragments

Remove ppc*-vxworks7* inadequate libgcc Makefile fragments

t-linux assigns .so version numbers to a set of
symbols, some of which aren't included the VxWorks libgcc
on powerpc (from ibm-ldouble.c, in particular).

t-slibgcc-libgcc yields a kind of .so file that the default
loader can't handle. This sort of extension to tmake_file for
shared libs will be better handled in a grouped fashion for
all targets anyway.

2021-12-13  Olivier Hainque  <hainque@adacore.com>

	* config.host (powerpc*-*-vxworks7*): Remove
	rs6000/t-linux and t-slibgcc-libgcc from tmake_file.