libstdc++: Use -fexcess-precision=standard for std::generate_canonical tests

libstdc++: Use -fexcess-precision=standard for std::generate_canonical tests

On 32-bit x86 the default -fexcess-precision=fast setting used by
-std=gnu++20 results in unpredictable rounding which alters the number
of times that std::generate_canonical produces exactly 1.0f, which
means that the URBG is invoked a different number of times.

To ensure that the behaviour is consistent and the expected number of
calls happens, use -fexcess-precision=standard.

libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/random/uniform_real_distribution/operators/64351.cc:
	Add -fexcess-precision=standard to options.
	* testsuite/26_numerics/random/uniform_real_distribution/operators/gencanon.cc:
	Likewise.

Reviewed-by: Jakub Jelinek <jakub@redhat.com>