meson.build: Define both CSP_BIG_ENDIAN and CSP_LITTLE_ENDIAN
meson.build: Define both CSP_BIG_ENDIAN and CSP_LITTLE_ENDIAN

In #219, we decided to define all configuration parameters.  This
commit make sure that both CSP_BIG_ENDIAN and CSP_LITTLE_ENDIAN are
defined when the Meson build system is used.

Signed-off-by: Yasushi SHOJI <afa9ca3be149447780c2f7b524d61ed3d3db63f9@spacecubics.com>

libcsp/libcsp,libcsp/libcsp