androidndk: fix make-standalone-toolchain.sh helper script

The script does a copy of a toolchain and then rearranges files a bit.
The problem is that these files have the same permissions (read-only)
as installed ones. The patch fixes the problem by changing permissions
of the copy before doing anything else.
Merge pull request #4268 from ftrvxmtrx/androidndk-fix-make-standalone-toolchain

androidndk: fix make-standalone-toolchain.sh helper script