[Ada] fix handling of Standard'Maximum_Alignment on x86_64

[Arnaud Charlet <charlet at adacore dot com>]

Manually tested on x86_64-linux.
Tested on i686-linux. Committed on mainline.

Currently there is rather massive confusion over the meaning of maximum
alignment on x86 windows, and GNAT has a rather nasty kludge which
attempts (but does not do a very good job) of trying to undo this gcc
confusion. Unfortunately the x86_64 got ensnared in this kludge, and this
patch undoes this so that we get the proper value of Maximum_Alignment (16)
on x86_64. A trivial test is the following which should output 16 on
x86_64:

with Text_IO; use Text_IO;
procedure K is
begin
   Put_Line (Integer'Image (Standard'Maximum_Alignment));
end;

2005-03-08  Robert Dewar  <dewar@adacore.com>

	* exp_smem.adb, sem_attr.adb: Remove OK_For_Stream flag, not used,
	not needed.
	Add documentation to replace the use of this flag
	Fix kludge for Maximum_Alignment on x86 so that it does not apply to
	the x86_64.

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