[Ada] Ensure consistency of fpt exponentiation results

[Arnaud Charlet <charlet at adacore dot com>]

This change ensures on all targets that A**B = A**C when B is a small
static constant in the range 0 .. 4 and C is a variable with the same
value. Previously for Float and Long_Float on some targets, this was
not the case. The results given were both within the error bounds that
are allowed by the Ada standard, but it is desirable not to have this
discrepancy.

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

2015-05-26  Robert Dewar  <dewar@adacore.com>

	* exp_ch4.adb (Wrap_MA): New function.
	(Expand_N_Op_Expon): Use Wrap_MA.

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