run-tests.py: Use sys.byteorder to make code more readable. NFC (#1855)
run-tests.py: Use sys.byteorder to make code more readable. NFC (#1855)

Split out from #1843.

This also avoids adding -DWABT_BIG_ENDIAN=0 pointlessly to all build commands on little endian machines.
WebAssembly/sexpr-wasm-prototype,WebAssembly/sexpr-wasm-prototype,WebAssembly/sexpr-wasm-prototype,WebAssembly/sexpr-wasm-prototype,WebAssembly/wabt,WebAssembly/wabt,WebAssembly/wabt,WebAssembly/sexpr-wasm-prototype,WebAssembly/wabt,WebAssembly/wabt,WebAssembly/sexpr-wasm-prototype