This test currently hardcodes which environments have got 80 bit long
doubles on x86_64 with a suitable printf formatting; convert the
architecture check into a check specifically for 80 bit long doubles.
Not all x86_64 configurations do have 80 bit long doubles (e.g.
Android doesn't), and i386 configurations can also have 80 bit long
doubles, compatible with this test.
Also clarify the exact reasons for why specific OSes such as FreeBSD are
skipped for these tests, even though they have 80 bit long doubles.
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