Provide Tor/obfsproxy arm binaries for Linux

Let's properly document all the ARM binary stuff for Briar Headless.

Briar Headless currently only includes binaries for linux-x86_64. While Briar Android works on ARM, we need a special binary for (plain) Linux systems and enable them in code.

Here is a table with required flags for specified target devices:

command	-march=	other flags	target devices
arm-linux-gnueabi-gcc	armv6	-marm	Raspberry Pi Model B Rev 2
arm-linux-gnueabi-gcc	armv6zk	-marm	Raspberry Pi Model B Rev 2
arm-linux-gnueabi-gcc	armv6zk+fp	-marm	Raspberry Pi Model B Rev 2
arm-linux-gnueabihf-gcc	armv7-a+fp	-mmusl	Google Nexus 5 (postmarketOS)
arm-linux-gnueabihf-gcc	armv7ve+simd	-marm	Raspberry Pi 2 Model B (not tested)
arm-linux-gnueabihf-gcc	unknown	unknown	Raspberry Pi Model 3 (not tested)
aarch64-linux-gnu-gcc	armv8-a+crc+simd	-marm	Raspberry Pi 4 Model B (not tested)
aarch64-linux-gnu-gcc	armv8-a+crypto+crc	-	Purism Librem 5 (not tested)

To add information on another device, the following commands are helpful:

cat /proc/cpuinfo
gcc -c -Q -march=native --help=target
gcc -v

Python also gives interesting information (enter these commands after starting python3; you can exit with ctrl + d):

import platform
platform.architecture()
platform.machine() 

For detecting your device properly, we depend on Java to tell us which is it:

public class Arch {
    public static void main(String[] args) {
        System.out.println(System.getProperty("os.arch"));
    }
}

Please compile it with javac Arch.java and post the output of java Arch.

Once you have that information, feel free to post the output to this issues. Your work will help getting Briar run on your device.

You can further help by trying to do a test compile. Using the following briar.c...

#include <stdio.h>

int main() {
   float a = 3.33 + 6.66;
   printf("Briar rocks, %f!", a);
   return 0;
}

...you can compile the program using e.g. arm-linux-gnueabi-gcc test.c -o test.out -march=armv6zk+fp -marm. Under Debian, there are gcc-arm-linux-gnueabihf and gcc-arm-linux-gnueabi available as cross-compilers. Please compile the program on your desktop device using the right compiler flags, copy the test.out binary to your target device and try to execute it there. If it works, you can additionally provide the exact compile command that worked for you.