vars_spec('win-arm64', 'win32')
'arm64_x86'
>>> _get_vcvars_spec('win-arm64', 'win-amd64')
'arm64_amd64'

Otherwise, always cross-compile from x86 to work with the
lighter-weight MSVC installs that do not include native 64-bit tools.

>>> _get_vcvars_spec('win32', 'win32')
'x86'
>>> _get_vcvars_spec('win-arm32', 'win-arm32')
'x86_arm'
>>> _get_vcvars_spec('win-amd64', 'win-arm64')
'x86_arm64'
r2