testsuite: Make AArch64 attributes_7.c generate unique report lines.

testsuite: Make AArch64 attributes_7.c generate unique report lines.

This test is executed in both C and C++ modes, lines 98 and 100 pass
in C++ and are xfail in C.

This results in similar lines in gcc.sum and g++.sum, differing by the
PASS or XFAIL prefix, which confuses compare_test: it reports these
tests twice, as "now fail, but worked before" and "now work, but
didn't before". To make the diagnostic different betweek C and C++, I
use $tool in the dg-error comment field.

2022-05-13  Christophe Lyon  <christophe.lyon@arm.com>

	gcc/testsuite/
	* gcc.target/aarch64/sve/acle/general/attributes_7.c: Make
	diagnostic unique.