MDEV-34970 Vector search fails to compile on s390x

add missing casts to float4store/float8store for bigendian.
fix a typo in float4store() usage
remove unnecessary one-byte-at-time appends

include/big_endian.h

@@ -16,7 +16,7 @@
 /*
   Data in big-endian format.
 */
-#define float4store(T,A) do { *(T)= ((uchar *) &A)[3];\
+#define float4store(T,A) do { *((T)+0)=(char) ((uchar *) &A)[3];\
                               *((T)+1)=(char) ((uchar *) &A)[2];\
                               *((T)+2)=(char) ((uchar *) &A)[1];\
                               *((T)+3)=(char) ((uchar *) &A)[0]; } while(0)
@@ -28,7 +28,7 @@
                               ((uchar*) &def_temp)[3]=(M)[0];\
                               (V)=def_temp; } while(0)
 
-#define float8store(T,V) do { *(T)= ((uchar *) &V)[7];\
+#define float8store(T,V) do { *((T)+0)=(char) ((uchar *) &V)[7];\
                               *((T)+1)=(char) ((uchar *) &V)[6];\
                               *((T)+2)=(char) ((uchar *) &V)[5];\
                               *((T)+3)=(char) ((uchar *) &V)[4];\

sql/item_vectorfunc.cc

@@ -179,11 +179,8 @@ String *Item_func_vec_fromtext::val_str(String *buf)
             goto error_format;
 
         char f_bin[4];
-        float4store(&f_bin, f);
-        buf->append(f_bin[0]);
-        buf->append(f_bin[1]);
-        buf->append(f_bin[2]);
-        buf->append(f_bin[3]);
+        float4store(f_bin, f);
+        buf->append(f_bin, sizeof(f_bin));
         break;
       }
       default: