add support for armv7l (ignores armv6l)

[bhamail]

Summary
Add support for armv7l (ignores armv6l) for "arm" architecture (as detected by Java system property "os.arch").

Related to issue #704
This PR adds support for installing node on armv7l on a Raspberry Pi.

On the LTS node download site: https://nodejs.org/dist/v8.10.0/
there are only two choices for "arm" versions of node ("aarch64" is a different case): armv6l and armv7l. This patch simply uses the newer v7 flavor.

Tests and Documentation

I ran the following test code on a raspberry pi to confirm the behavior produced valid node download URLs:

package com.github.eirslett.maven.plugins.frontend.lib;

public class PlatformTest
{
  public static void main(final String[] args) {
    final com.github.eirslett.maven.plugins.frontend.lib.Platform platform = Platform.guess();

    System.out.println("os.arch:" + System.getProperty("os.arch"));
    final Architecture architecture = Architecture.guess();
    System.out.println("Architecture.guess(): " + architecture);

    final String actual = platform.getNodeDownloadFilename("v8.10.0", false);
    System.out.println("Platform.getNodeDownloadFilename(): " + actual);
  }
}

The test output was:

os.arch:arm
Architecture.guess(): armv7l
Platform.getNodeDownloadFilename(): v8.10.0/node-v8.10.0-linux-armv7l.tar.gz

That output should produce a valid download URL from this page:

https://nodejs.org/dist/v8.10.0/