fixed large code duplication in write_new_stack methods

git-svn-id: http://svn.code.sf.net/p/bochs/code/trunk@12497 6c8e4198-a580-4521-a02f-c9ae86df1db9
removed duplication in XCHG instruction handlers

git-svn-id: http://svn.code.sf.net/p/bochs/code/trunk@12498 6c8e4198-a580-4521-a02f-c9ae86df1db9