use `uname -m` instead of `uname -p` (#593)

`uname -p` is unportable on some platforms.
Signed-off-by: Yang Keao <keao.yang@yahoo.com>
Co-authored-by: Xinye Tao <xy.tao@outlook.com>

scripts/generate-bindings.sh

@@ -6,7 +6,7 @@
 
 export UPDATE_BIND=1
 if [ "$ARCH" == "" ]; then
-    ARCH=`uname -p`
+    ARCH=`uname -m`
 fi
 cargo build  --target ${ARCH}-unknown-linux-gnu
 rustfmt librocksdb_sys/bindings/*