tests: Avoid 64-bit arithmetic in assembler

For testing we (ab)use the assembler to build us a sample dtb, independent
of the other tools (dtc and libfdt) that we're trying to test.  In a few
places this uses 64-bit arithmetic to decompose 64-bit constants into
the individual bytes in the blob.

Unfortunately, it seems that some builds of GNU as don't support >32 bit
arithmetic, though it's not entirely clear to me which do and which don't
(Fedora i386 does support 64-bit, Debian arm32 doesn't).

Anyway, to be safe, this avoids 64-bit arithmetic in assembler at the cost
of some extra awkwardness because we have to define the values in 32-bit
halves.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>