Add "x86_64" and "x86-64" aliases for "x64" (#44)

By a popular request...

README.md

@@ -56,7 +56,7 @@ jobs:
 
 - `arch` – target architecture
   - native compilation:
-    - `x64` (default) or its synonyms: `amd64`, `win64`
+    - `x64` (default) or its synonyms: `amd64`, `win64`, `x86_64`
     - `x86` or its synonyms: `win32`
   - cross-compilation: `x86_amd64`, `x86_arm`, `x86_arm64`, `amd64_x86`, `amd64_arm`, `amd64_arm64`
 - `sdk` – Windows SDK to use

index.js

@@ -90,6 +90,8 @@ function main() {
     let arch_aliases = {
         "win32": "x86",
         "win64": "x64",
+        "x86_64": "x64",
+        "x86-64": "x64",
     }
     // Ignore case when matching as that's what humans expect.
     if (arch.toLowerCase() in arch_aliases) {