Unify linking of libasm, libelf, libdw, backends

Unify linking of libasm, libelf, libdw, backends

Link them all with -z,defs,-z,relro,--no-undefined, provide complete
dependencies for the link steps, and add libeu.a to each one. libeu.a
contains useful library functionality that each of them might use. The
linker will strip unneeded symbols, so linking it in won't hurt even if
none of the functions are used.

Signed-off-by: Ulf Hermann <ulf.hermann@qt.io>