Only strip files under the bin directory (WebAssembly#275)

Stripping other files (eg. *.a) doesn't seem the original intention
and it sometimes has ill effects like breaking archive TOC.

Author: yamt

Makefile

@@ -200,7 +200,7 @@ build/config.BUILT:
 build: build/llvm.BUILT build/wasi-libc.BUILT build/compiler-rt.BUILT build/libcxx.BUILT build/config.BUILT
 
 strip: build/llvm.BUILT
-	./strip_symbols.sh $(BUILD_PREFIX)
+	./strip_symbols.sh $(BUILD_PREFIX)/bin
 
 package: build/package.BUILT