sandbox: Detect the host bit size automatically

At present if you build sandbox on a 32-bit host a lot of errors are
produced. This is because CONFIG_HOST_64BIT is enabled by default.

It is quite annoying to have to change that manually before building
sandbox. It is also quite confusing for new users.

Add a way to detect the setting and add the appropriate
CONFIG_HOST_64BIT=y or CONFIG_HOST_32BIT=y to the defconfig, to avoid
this issue.

Tidy up the Kconfig help for the above two options while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>