pack external32 long double conversion (extended 80 / quad 128)

On architectures that store long doubles as 80 bit extended precisions
or as 64 bit "float64"s, we need conversions to 128 bit quad precision to
satisfy MPI_Pack_external/Unpack_external.  I added a couple more
arguments to pFunction to know what architecture the 'to' and 'from'
buffers are.  Previously we had architecture info 'local' and 'remote'
but I don't know how to correlate local/remote with to/from without
adding more arguments as I did.

With the incresed information about the context, the conversion function
can now convert the long double as needed.

I'm using code Lisandro Dalcin contributed for the floating point
conversions in f80_to_f128, f64_to_f128, f128_to_f80, and f128_to_f64.
These conversion functions require the data to be in local endianness,
but one of the sides in pack/unpack is always local so operations can
be done in an order that allows the long double conversion to see the
data in local endianness.

I also added a path to use __float128 for the conversion
for #ifdef HAVE___FLOAT128 as that ought to be the more reliable
method than rolling our own bitwise conversions.

The reason for all the arch.h changes is the former code was
inconsistent as to how bits were labeled within a byte, and had
masks like LONGISxx that didn't match the bits they were supposed
to contain.

Signed-off-by: Mark Allen <[email protected]>

Author: markalle