Makefile: change STRIP to '--strip-program' when cross-compile

When cross-compile, we should use specific strip instead of default
/usr/bin/strip.

reference:https://man7.org/linux/man-pages/man1/install.1.html

Signed-off-by: mingrui.ren <mingrui.ren@bst.ai>
diff --git a/Makefile b/Makefile
index 3df076d..9431461 100644
--- a/Makefile
+++ b/Makefile
@@ -45,10 +45,11 @@
 # Commands
 INSTALL=install
 DIRINSTALL=install -d
-STRIP=-s
 ifdef CROSS_COMPILE
+STRIP=--strip-program $(CROSS_COMPILE)-strip
 CC=$(CROSS_COMPILE)gcc
 else
+STRIP=-s
 CC=cc
 endif
 AR=$(CROSS_COMPILE)ar