`nvm_get_os` also returns `win` (Cygwin/MSYS/MinGW) and the empty string,
neither of which had an arm,
so `NVM_CPU_CORES` went unassigned
and nvm asked the user to report a gap it already knew about.
On an unrecognized `uname` that also meant a single-threaded source build;
`win` refuses source builds a few lines later either way.
`NUMBER_OF_PROCESSORS` comes last because Windows scopes it to the calling process' processor group.
Assigning `NVM_CPU_CORES` in every arm also keeps the
`nvm_is_natural_num` check below from aborting dash and ksh under `set -u`.
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