Packages/Rts: add `linker` to RTS directories

Authored by michalt on May 25 2016, 1:14 PM.

Description

Packages/Rts: add linker to RTS directories

Recent commit split off the m32 allocator to rts/linker/, which
broke the build using Hadrian (since it didn't know about the new
directory). This fixes it.

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>