This change adds the ability to produce host static build for the
libdouble-conversion library. Other host build tools can now depend on
this library if they need it.

Tested on a x86_64 host build machine.
make package/libdouble-conversion/host/compile produces
staging_dir/hostpkg/lib/libdouble-conversion.a file

Signed-off-by: Amol Bhave <ambhave@fb.com>