[Ada] Makes the exception type a little bit more general

[Arnaud Charlet <charlet at adacore dot com>]

The OpenVMS specific component Import_Code is replaced by Foreign_Data.
Although the size of the component is not the same, due to alignment constraint
of the component before and after, the size of the record doesn't change.
Furthermore, this component is never used by the code generated for gnat1 or
gnatbind, so there is no bootstrap issue.

No functional change, so no new testcase.

Tested on x86_64-pc-linux-gnu, committed on trunk

2013-10-14  Tristan Gingold  <gingold@adacore.com>

	* cstand.adb (Create_Standard): Change Import_Code component
	of Standard_Exception_Type to Foreign_Data. Its type is now
	Standard_A_Char (access to character).
	* exp_prag.adb (Expand_Pragma_Import_Export_Exception): Adjust
	definition of Code to match the type of Foreign_Data.
	* s-stalib.ads (Exception_Data): Replace Import_Code by Foreign_Data
	Change the definition of standard predefined exceptions.
	(Exception_Code): Remove.
	* raise.h (Exception_Code): Remove (Exception_Data): Replace
	Import_Code field by Foreign_Data.
	* rtsfind.ads (RE_Exception_Code): Remove
	(RE_Import_Address): Add.
	* a-exexpr-gcc.adb (Import_Code_For): Replaced by Foreign_Data_For.
	* exp_ch11.adb (Expand_N_Exception_Declaration): Associate null
	to Foreign_Data component.
	* raise-gcc.c (Import_Code_For): Replaced by Foreign_Data_For.
	(is_handled_by): Add comments. Use replaced function. Change
	condition so that an Ada occurrence is never handled by
	Foreign_Exception.
	* s-exctab.adb (Internal_Exception): Associate Null_Address to
	Foreign_Data component.
	* s-vmexta.adb, s-vmexta.ads (Exception_Code): Declare Replace
	SSL.Exception_Code by Exception_Code.

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