speaker-test: only check byte order once

speaker-test: only check byte order once

Rather than having numerous preprocessor directives scattered in the code
checking __BYTE_ORDER, only check it once and define a set of macros
accordingly that can be used in the rest of the code. This makes things
simpler to read and less error-prone.

Signed-off-by: Dan McGee <dpmcgee@gmail.com>