gmp: add host compile

gmp: add host compile

Currently, any package that uses host compile and depends on
libgmp.so will fail. This is because gmp is not compile for
host machine. So, staging_dir/host has only static lib for gmp

 $ ls staging_dir/host/lib/libgmp* -1
 staging_dir/host/lib/libgmp.a
 staging_dir/host/lib/libgmp.la
 staging_dir/host/lib/libgmpxx.a
 staging_dir/host/lib/libgmpxx.la

Addind host compile in gmp, the dependent package can use:

 PKG_BUILD_DEPENDS:=gmp/host

That will compile gmp to staging_dir/host with *.so files

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Backport of r42196

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42369 3c298f89-4303-0410-b956-a3cf2f4a3e73