config: Use CONFIG_SYS_BIG_ENDIAN in code whenever possible

So CONFIG_SYS_BIG_ENDIAN is our cross architecture option for
selecting machine endian, while the old CONFIG_CPU_BIG_ENDIAN
is defined by Arc only.

Use it whenever possible to ensure big endian code path is enabled
for all possible big endian machines.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>