adding arm64 build, and improved termux installation process

* Added arm64 Linux standalone build. (No autobuilder yet.)
* Improved termux installation process.

Added git-annex-install.sh script to avoid user needing to type as much in
termux. The scope of this script is limited; runshell handles the rest.

Runshell runs termux-fix-shebang on the shell scripts. The problem is
the bundled bin/sh script, deleting that script also works, but then the
others probably use the system Android /bin/sh, which could be old or
broken or not posix or whatever. Using termux sh to run the scripts is
better.

This commit was sponsored by Eric Drechsel on Patreon.

standalone/linux/skel/runshell

@@ -189,6 +189,8 @@ case "$os" in
 			# it and use termux's git which works better.
 			cd "$base"
 			find . | grep git | grep -v git-annex | grep -v git-remote-tor-annex | grep -v git-remote-gcrypt | xargs rm -rf
+			# Fix shell scripts to work when run inside proot.
+			termux-fix-shebang bin/* runshell git-annex git-annex-shell git-annex-webapp 
 			cd "$orig"
 			# Save the poor Android user the typing.
 			if ! [ -e "$HOME/.profile" ] || ! grep -q "$base" "$HOME/.profile"; then