Fix ILP32 data model detection with GCC on 32bit ARM

Unfortunately, GCC does not define `__ILP32__` on 32 bit ARM (contrary
to e.g. x86), so rely on `__arm__` instead.

This does not change API/ABI, but fixes a compilation error for one
specific architecture and compiler combination.

Avoids broken workarounds in every downstream user of
mfxdefs.h,  e.g.:  
intel/libvpl@6182082a12#diff-1e7de1ae2d05  
intel/cartwheel-ffmpeg#215

Author: StefanBruens

source/elements/oneVPL/include/vpl/mfxdefs.h

@@ -68,7 +68,7 @@ extern "C"
     #define MFX_PACK_BEGIN_STRUCT_W_PTR()    MFX_PACK_BEGIN_X(4)
     #define MFX_PACK_BEGIN_STRUCT_W_L_TYPE() MFX_PACK_BEGIN_X(8)
 /* 32-bit ILP32 data model Linux* */
-#elif defined(__ILP32__)
+#elif defined(__ILP32__) || defined(__arm__)
     #define MFX_PACK_BEGIN_STRUCT_W_PTR()    MFX_PACK_BEGIN_X(4)
     #define MFX_PACK_BEGIN_STRUCT_W_L_TYPE() MFX_PACK_BEGIN_X(4)
 #else