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 2c713d6e (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.