Clean up icc + ia64 situation.

 Clean up icc + ia64 situation.
 
 Some googling turned up multiple sources saying that older versions of icc
 do not accept gcc-compatible asm blocks on IA64, though asm does work on
 x86[_64].  This is apparently fixed as of icc version 12.0 or so, but that
 doesn't help us much; if we have to carry the extra implementation anyway,
 we may as well just use it for icc rather than add a compiler version test.
 
 Hence, revert commit 2c713d6ea29c91cd2cbd92fa801a61e55ea2a3c4 (though I
 separated the icc code from the gcc code completely, producing what seems
 cleaner code).  Document the state of affairs more explicitly, both in
 s_lock.h and postgres.c, and make some cosmetic adjustments around the
 IA64 code in s_lock.h.