context: new layout for fcontext_t

- fcontext_t takes pointer to stack (lower addresses) and stack size
 - make_fcontext() computes start address of stack pointer depending on
   architecture (growing upwards, downwards)
 - make_fcontext() preserves frame pointer on intel
 - stack alignment corrected on intel
 - BOOST_USE_UCONTEXT for performance measurements added


[SVN r80338]

example/transfer.cpp

@@ -32,7 +32,7 @@ int main( int argc, char * argv[])
 {
     ctx::stack_allocator alloc;
 
-    fc1.fc_stack.base = alloc.allocate(ctx::minimum_stacksize());
+    fc1.fc_stack.sp = alloc.allocate(ctx::minimum_stacksize());
     fc1.fc_stack.size = ctx::minimum_stacksize();
     ctx::make_fcontext( & fc1, f1);