SSIZE_MAX on ia64

[Roland McGrath]

SSIZE_MAX must accurately reflect the ssize_t type.  It is presently
incorrect on all 64-bit platforms.  On every platform I am aware of,
ssize_t == long int (i.e. 32 bits where long is 32 and 64 bits where long
is 64).  So it would not hurt to generically use LONG_MAX, and that would
be simpler.  I think it is a valid assumption at the moment that long ==
wordsize.  If not, we could also have SSIZE_MAX defined conditional on
__WORDSIZE, which is still simpler than adding a specific definition for
particular platforms.