context: fixes for PPC

svn path=/trunk/libs/context/; revision=83313
context: fix for bug #8544 (Win32 only)

svn path=/trunk/boost/context/; revision=84133
context: fix deallocation stack issue on 64bit Windows

svn path=/trunk/libs/context/; revision=84181
context: add hint for make_fcontext() in docu

svn path=/trunk/libs/context/; revision=84555