libgcc: remove crt{begin,end}.o from powerpc-wrs-vxworks target

Since commit 78e49fb1 (Introduce vxworks specific crtstuff support),
the generic crtbegin.o/crtend.o have been unnecessary to build. So
remove them from extra_parts.

This is effectively a revert of commit 9a5b8df7 (libgcc: add
crt{begin,end} for powerpc-wrs-vxworks target).

libgcc/
	* config.host (powerpc-wrs-vxworks): Do not add crtbegin.o and
	crtend.o to extra_parts.