[sdk] Use os.uname() to figure out the host architecture on macOS

platform.machine() might return x86_64 if run under rosetta causing the
script to assume that we're building x64 instead of arm64.

Change-Id: I169890f41ca71b08f3068da609cbe0579743051b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/202770
Reviewed-by: William Hesse <whesse@google.com>

tools/bots/dart_sdk.py

@@ -24,7 +24,8 @@ def BuildArchitectures():
     if BUILD_OS == 'linux':
         return ['ia32', 'x64', 'arm', 'arm64']
     elif BUILD_OS == 'macos':
-        return [BUILD_ARCHITECTURE]
+        arch = 'arm64' if 'arm64' == os.uname().machine else 'x64'
+        return [arch]
     else:
         return ['ia32', 'x64']