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>
diff --git a/Makefile b/Makefile
index f5b2512..aa79142 100644
--- a/Makefile
+++ b/Makefile
@@ -1048,7 +1048,7 @@
 CHECKFLAGS += --arch=$(ARCH)
 
 # insure the checker run with the right endianness
-CHECKFLAGS += $(if $(CONFIG_CPU_BIG_ENDIAN),-mbig-endian,-mlittle-endian)
+CHECKFLAGS += $(if $(CONFIG_SYS_BIG_ENDIAN),-mbig-endian,-mlittle-endian)
 
 # the checker needs the correct machine size
 CHECKFLAGS += $(if $(CONFIG_64BIT),-m64,-m32)