ghc.mk: don't build gmp packages for BIGNUM_BACKEND=native

ghc.mk: don't build gmp packages for BIGNUM_BACKEND=native

Before this change make-based BIGNUM_BACKEND=native build was failing as:

x86_64-pc-linux-gnu-gcc: error: libraries/ghc-bignum/gmp/objs/*.o: No such file or directory

This happens because ghc.mk was pulling in gmp-dependent haskell libraries unconditionally. The change avoid building such libraries.

Bug: #18437 (closed)