Default to ARM64 when running on Arm hosts

If the function architecture isn't specified and we're running on an Arm
platform assume we should be trying to run the function for ARM64

Fixes #4708

samcli/lib/providers/provider.py

@@ -7,6 +7,7 @@
 import os
 import posixpath
 from collections import namedtuple
+from platform import machine
 from typing import Any, Set, NamedTuple, Optional, List, Dict, Tuple, Union, cast, Iterator, TYPE_CHECKING
 
 from samcli.commands.local.cli_common.user_exceptions import (
@@ -20,7 +21,7 @@
     SAM_METADATA_SKIP_BUILD_KEY,
     SAM_RESOURCE_ID_KEY,
 )
-from samcli.lib.utils.architecture import X86_64
+from samcli.lib.utils.architecture import X86_64, ARM64
 
 if TYPE_CHECKING:  # pragma: no cover
     # avoid circular import, https://docs.python.org/3/library/typing.html#typing.TYPE_CHECKING
@@ -129,7 +130,7 @@ def architecture(self) -> str:
             If the architectures value is invalid
         """
         if not self.architectures:
-            return X86_64
+            return ARM64 if machine() in ("arm64", "aarch64") else X86_64
 
         arch_list = cast(list, self.architectures)
         if len(arch_list) != 1:

tests/unit/commands/local/lib/test_provider.py

@@ -1,5 +1,6 @@
 import os
 import posixpath
+from platform import machine
 from unittest import TestCase
 from unittest.mock import MagicMock, Mock, patch
 
@@ -300,7 +301,7 @@ def setUp(self) -> None:
     @parameterized.expand(
         [
             ([ARM64], ARM64),
-            ([], X86_64),
+            ([], ARM64 if machine() in ("arm64", "aarch64") else X86_64),
             ([X86_64], X86_64),
         ]
     )