(alpha_sa_size): Need to save $26 if it is used or if any other
register is saved, but not just because a stack space is needed.
(output_prolog): Add code to probe the stack if we allocate more than
4096 bytes; required by chapter 7 (for threads).
Know when to save $26.
(output_epilog): Know when to restore $26.

From-SVN: r4079