nettle: add mini-gmp support

Signed-off-by: Stephan Raue <stephan@openelec.tv>
gnutls: enable nettle-mini support

Signed-off-by: Stephan Raue <stephan@openelec.tv>
gmp: remove target build support

Signed-off-by: Stephan Raue <stephan@openelec.tv>