expression: fix constant expression allocation on big endian

expression: fix constant expression allocation on big endian

When allocating a constant expression, a pointer to the data is passed
to the allocation function. When the variable used to store the data
is larger than the size of the data type, this fails on big endian since
the most significant bytes (being zero) come first.

Add a helper function to calculate the proper address for the cases
where this is needed.

This currently affects symbolic tables for values < u64 and payload
dependency generation for protocol values < u32.

Signed-off-by: Patrick McHardy <kaber@trash.net>