stdio-common: Add scanf long double data for Intel/Motorola 80-bit format

stdio-common: Add scanf long double data for Intel/Motorola 80-bit format

Add Makefile infrastructure, a format-specific test skeleton providing a
data comparison implementation that ignores bits of data representation
in memory that do not participate in holding floating-point data, and
`long double' real input data for targets using the Intel/Motorola
80-bit format.

Keep input data disabled and referring to BZ #12701 for entries that are
are currently incorrectly accepted as valid data, such as '0e', '0e+',
'0x', '0x8p', '0x0p-', etc.

Reviewed-by: Joseph Myers <josmyers@redhat.com>