Read/write external resource alignment tag in little-endian

https://reviews.llvm.org/D126446 added support for encoding
binary blobs in MLIR assembly.  To enable cross-architecture
compatibility, these need to be encoded in little-endian format.

This patch is a first step in that direction by reading and
writing the alignment tag that those blobs are prefixed by
in little-endian format.  This fixes assertion failures in
several test cases on big-endian platforms.

The actual content of the blob is not yet handled here.

Differential Revision: https://reviews.llvm.org/D129483