[Ada] Fix inconsistent usage of Machine in s-fatgen.adb

[Pierre-Marie de Rodat <derodat at adacore dot com>]

System.Fat_Gen is a generic unit implementing support routines for floating-
point attributes, for example the 'Machine attribute.  These routines make
themselves use of the 'Machine attribute, some of them by calling the
Machine support routine directly, some others by using the attribute.

Consistency dictates that a single idiom be used and the latter is to be
preferred, since it generates better code for targets without excessive
precision issues, i.e. all of them except for x86 and x86-64.

No functional changes.

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

2017-12-15  Eric Botcazou  <ebotcazou@adacore.com>

	* exp_attr.adb (Is_Inline_Floating_Point_Attribute): Fix comment.
	* libgnat/s-fatgen.adb (Model): Use Machine attribute.
	(Truncation): Likewise.

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