choosenim breaks user code on M1 Mac
choosenim seems to break packages and user code when installing Nim on an M1 mac  (Apple silicone, ARM64) . 

The issue seems to be that choosenim compiles the nim compiler as an x86 executable. Checking this, I get:
```
$ file `which nim`
/Users/mode80/.nimble/bin/nim: Mach-O 64-bit executable x86_64
```

When packages and user code make use of code like `when defined(arm64)`, 
it incorrectly reports false, and instead reports true for `defined(amd64)`.  The program usually crashes thereafter because whatever code path was needed for M1 doesn't run. (For example, the threading module using processor-specific assembly instructions based on this kind of check. ) 

When I install nim with homebrew, and test, I get 
```
$ file `which nim`
/opt/homebrew/bin/nim: Mach-O 64-bit executable arm64
```

Then things work as expected.
