Multiply by 2 instead of left-shifting a potentially negative number

fixes GCC ubsan warning in the BLAS tests

kernel/x86_64/zgemv_t_4.c

@@ -235,9 +235,9 @@ int CNAME(BLASLONG m, BLASLONG n, BLASLONG dummy1, FLOAT alpha_r, FLOAT alpha_i,
         if ( m < 1 ) return(0);
         if ( n < 1 ) return(0);
 
-        inc_x <<= 1;
-        inc_y <<= 1;
-        lda   <<= 1;
+        inc_x *= 2;
+        inc_y *= 2;
+        lda  <<= 1;
 	lda4    = lda << 2;
 
 	xbuffer = buffer;