Improve sequence logic in cxx_init_decl_processing

Improve sequence logic in cxx_init_decl_processing

Checking for one_only/weak support is better done
before deciding to turn references to __cxa_pure_virtual weak.

This helps at least on VxWorks where one_only / weak support
varies between kernel and rtp modes as well as across VxWorks
versions.

2021-12-30  Olivier Hainque  <hainque@adacore.com>

gcc/cp/
	* decl.c (cxx_init_decl_processing): Move code possibly
	altering flag_weak before code testing it.