target-i386: Introduce feat2prop() for CPU properties

This helper replaces '_' with '-' in a uniform way.
As a side effect, even custom mappings must use '-' now.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
[AF: Operate on NUL-terminated string rather than '=' delimiter]
Signed-off-by: Andreas Färber <afaerber@suse.de>