[llvm-jitlink] Fix a bug in llvm-jitlink's Slab allocator.

The slab delta (used to link as if allocated at a specified address) should
remain constant.The update to the delta was accidentally introduced in
962a2479, but hasn't caused any failures as it only breaks in an obvious
way for multi-file exec uses (our regression tests are all -noexec, and tend to
be single-file).

No testcase here: this is an obscure utility for testing support, and an
uncommon use-case. If the slab allocator is ever moved into LLVM we could add
a unit test to catch this.