rt: Save and restore %rax/%eax in __morestack

This doesn't matter now since we use an out pointer for return
values but it's sure to show up mysteriously someday.

src/rt/arch/i386/morestack.S

@@ -198,11 +198,17 @@ MORESTACK:
 	// Realign stack - remember that __morestack was called misaligned
 	subl $12, %esp
 
+	// Save the return value of the function we allocated space for
+	movl %eax, (%esp)
+
 	// Now that we're on the return path we want to avoid
 	// stomping on %eax. FIXME: Need to save and restore %eax to
 	// actually preserve it across the call to delete the stack
 	call UPCALL_DEL_STACK
 
+	// And restore it
+	movl (%esp), %eax
+
 	addl $12,%esp
 
 	popl %ebp

src/rt/arch/x86_64/morestack.S

@@ -100,8 +100,8 @@ MORESTACK:
 	// Switch back to the rust stack
 	movq %rbp, %rsp
 
-	// Align the stack again
-	pushq $0
+	// Save the return value
+	pushq %rax
 
 	// FIXME: Should preserve %rax here
 #ifdef __APPLE__
@@ -111,7 +111,7 @@ MORESTACK:
 	call UPCALL_DEL_STACK@PLT
 #endif
 
-	addq $8, %rsp
+	popq %rax // Restore the return value
 	popq %rbp
 	// FIXME: I don't think these rules are necessary
 	// since the unwinder should never encounter an instruction