dev-util/mingw64-runtime: switch to --prefix=/usr (from --prefix=/)

dev-util/mingw64-runtime: switch to --prefix=/usr (from --prefix=/)

A couple of changes:
- Allow dev-util/mingw64-runtime[headers-only] for cross-building case.
  One day crossdew won't need cross-${CTARGET}/mingw64-runtime packages
  and will be able to use dev-util/mingw64-runtime directly.
  This change is a step in that direction.
- Awitch to --prefix=/usr (from --prefix=/). This allow cross-building
  dev-util/mingw64-runtime itself and installing libraries into /usr/lib
  (instead of /lib).

Tested as:
- generated working toolchain:
  $ crossdev -t x86_64-w64-mingw32
- cross-built mingw64-runtime successfully:
  $ x86_64-w64-mingw32-emerge -v1 dev-util/mingw64-runtime

Package-Manager: Portage-2.3.24, Repoman-2.3.6