pkgimages: storage of link_ids should be compressed

[timholy]

Towards the end of writing out pkgimages, we write a section of link_ids:

julia/src/staticdata.c

Lines 2477 to 2484 in 1e5fdb2

	write_uint32(f, jl_array_len(s.link_ids_gctags));
	ios_write(f, (char*)jl_array_data(s.link_ids_gctags), jl_array_len(s.link_ids_gctags)*sizeof(uint64_t));
	write_uint32(f, jl_array_len(s.link_ids_relocs));
	ios_write(f, (char*)jl_array_data(s.link_ids_relocs), jl_array_len(s.link_ids_relocs)*sizeof(uint64_t));
	write_uint32(f, jl_array_len(s.link_ids_gvars));
	ios_write(f, (char*)jl_array_data(s.link_ids_gvars), jl_array_len(s.link_ids_gvars)*sizeof(uint64_t));
	write_uint32(f, jl_array_len(s.link_ids_external_fnvars));
	ios_write(f, (char*)jl_array_data(s.link_ids_external_fnvars), jl_array_len(s.link_ids_external_fnvars)*sizeof(uint64_t));

These store the buildid of the module for an external linkage; they act as an invariant name for a module we depend on, and are used for pointer relocation when we deserialize.

It turns out these are quite repetitious, and the large majority of such linkages are against the sysimage with buildid 0. Consequently we should be able to store them with some form of compression. This will only modestly shrink the size of pkgimages (maybe 5%ish), but it seems worth having.