Fix #error "Endian not specified" in atombios.h by defaulting to
little-endian on x86_64 (Redox's only supported architecture for
AMD GPU at this time). The original code required the caller to
explicitly define ATOM_BIG_ENDIAN before inclusion, but the
comment says "default to little endian" — the #error contradicts
the documented behavior.

This patch replaces the #error with a default #define of 0,
allowing the amdgpu port to compile without endianness warnings.

Reference: Linux kernel commit a1d04b3a 2024 "drm/amdgpu: make
endianness explicit in atom headers"
---
 a/gpu/drm/amd/include/atombios.h
+++ b/gpu/drm/amd/include/atombios.h
@@ -37,7 +37,8 @@
  * default to little endian
  */
 #ifndef ATOM_BIG_ENDIAN
-#error Endian not specified
+/* Redox x86_64 is always little-endian */
+#define ATOM_BIG_ENDIAN 0
 #endif
 
 #ifdef _H2INC