get-arch: improve Apple Silicon detection with uname -v

[Coeur]

uname -a is the same as uname -m -n -r -s -v, but the only relevant part that needs to be compared against ARM64 is the -v one.

This is especially important in case the name of the machine (uname -n) contains the string ARM64 (don't ask me why someone would do that).

➜  ~ uname -a
Darwin MacBook-Pro-WITH-A-WEIRD-NAME.local 24.0.0 Darwin Kernel Version 24.0.0: Wed Aug  7 03:08:06 PDT 2024; root:xnu-11215.1.9~22/RELEASE_ARM64_T6031 x86_64
➜  ~ uname -v
Darwin Kernel Version 24.0.0: Wed Aug  7 03:08:06 PDT 2024; root:xnu-11215.1.9~22/RELEASE_ARM64_T6031

Once this PR is merged, don't forget @balupton to update your Stack Overflow answer at https://stackoverflow.com/a/68148776 to replace uname -a with uname -v. Although, I've added my own answer too: https://stackoverflow.com/a/78891773

[balupton]

LGTM. I'll fix the tests this week then merge. Happy to have you here.

[balupton]

There needs to be more work to get the tests going. I'll fix that up later. Turns out the help outputs were also wrong, so I've fixed that. Merging, thank you.