LLVMJIT: Adapt to API changes in gdb and perf support.

During the work of upstreaming my previous patches for gdb and perf
support the API changed. Adapt.  Normally this wouldn't necessarily be
something to backpatch, but the previous API wasn't upstream, and at
least the gdb support is quite useful for debugging.

Author: Andres Freund
Backpatch: 11, where LLVM based JIT support was added.