32-bit and 64-bit SPARC ABIs have 64-bit integer data types.
SPARC, like many RISC architectures, also has natural alignment requirements. Attempting to dereference a pointer to a 4-byte-sized object requires 4-byte alignment, for example. 2-byte-sized objects require 2-byte alignment. 8-byte-sized objects require 8-byte alignment.
siphash24 is encountering this bug on modern SPARC (32-bit ABI currently, haven't tried compiling as 64-bit yet). The code simply is not portable.
Benjamin's patch gets the failing self-test (test_plistlib) to pass as well as the simple test case in msg275493 above.