darwin: remove 64-bit build detection on 32-bit OS

darwin: remove 64-bit build detection on 32-bit OS

A workaround added in early days of 64-bit OSX forced x86_64 if the
host machine had 64-bit support. This creates issues when cross-
compiling for ARM64. Additionally, the user can always use --cpu=* to
manually set the host CPU and therefore this workaround should be
removed.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Joelle van Dyne <j@getutm.app>
Message-id: 20210126012457.39046-12-j@getutm.app
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>