mrhlpr.pipeline.get_arch…: add riscv64 and arm64

Add riscv64 for... well you know it, and also arm64 -> aarch64 for
aarch64-darwin systems similar to this patch:
https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C167000939722.18650.8735853729695007724-0@git.sr.ht%3E

mrhlpr/pipeline.py

@@ -11,9 +11,11 @@ def get_arch_alpine_native():
 
     mapping = {
         "aarch64": "aarch64",
+        "arm64": "aarch64",
         "armv6l": "armhf",
         "armv7l": "armv7",
         "i686": "x86",
+        "riscv64": "riscv64",
         "x86_64": "x86_64",
     }
     if machine in mapping: