…2-byte aligned

#58594 updated isa_encode() to use ISA's xor_gen() function as an optimisation when m=1. That PR missed the case where the data and coding buffers are not aligned to 32 bytes. Passing misaligned buffers to xor_gen() can result in a segmentation fault.

This PR checks the alignment of the buffers and only uses xor_gen() if the buffers are 32-byte aligned. I have also restored the byte_xor() function that was removed in 58594. If buffers are not aligned, then byte_xor() will be used instead of xor_gen().

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