Fix swiftlang#394, aarch64 download links broken (swiftlang#396)

Fixed aarch64 download links for the current stable release.

Motivation:

Currently the `x86_64` and `aarch64` downloads for the current release point both to the `x86_64` download artifact for the release.

Modifications:

Added a condition in the download template which appends the arch to the url if it's not the default `x86_64`

Result:

Download links for `aarch64` will point to `aarch64` downlaod artifacts and `x86_64` to the `x86_64` ones.

Signed-off-by: paulober <[email protected]>

Author: paulober

download/_build-platform.html

@@ -46,6 +46,10 @@
                     {% assign platform_name_url = platform.name | remove: '.' | remove: ' ' | downcase %}
                     {% assign platform_name = platform.name | remove : ' ' | downcase %}
                 {% endif %}
+                {% if arch != "x86_64" %}
+                    {% assign platform_name_url = platform_name_url | append: "-" | append: arch %}
+                    {% assign platform_name = platform_name | append: "-" | append: arch %}
+                {% endif %}
                 {% assign tag_downcase = include.platform.tag | downcase %}
                 <a href="https://download.swift.org/{{ tag_downcase }}/{{ platform_name_url }}/{{ include.platform.tag}}/{{ include.platform.tag}}-{{ platform_name }}.{{ package_extension }}">{{ arch }}</a>
                 <a href="https://download.swift.org/{{ tag_downcase }}/{{ platform_name_url }}/{{ include.platform.tag}}/{{ include.platform.tag}}-{{ platform_name }}.{{ package_extension }}.sig" title="PGP Signature" class="signature">Signature ({{ arch }})</a>