pmu_x86: Fix bug where machine code gets GC'd

Keep the arrays containing machine code alive by storing references to
them in a Lua table. Otherwise they will be garbage collected.

dynasm uses a GC callback to unmap memory that was used for generated
code and so the most likely consequence is a segfault. Here is how it
looks in dmesg:

  segfault at 7fe0d50de000 ip 00007fe0d50de000 sp 00007ffcd2c89cb8 error 14

where "error 14" means an error during instruction fetch.

This problem triggered immediately when using the pmu library with
non-trivial code under test (running an app network).

src/lib/pmu_x86.dasl

@@ -24,11 +24,15 @@ local dasm = require("dasm")
 
 local gen = {}
 
+-- Table keeping machine code alive to the GC.
+local anchor = {}
+
 -- Utility: assemble code and optionally dump disassembly.
 function assemble (name, prototype, generator)
    local Dst = dasm.new(actions)
    generator(Dst)
    local mcode, size = Dst:build()
+   table.insert(anchor, mcode)
    if debug then
       print("mcode dump: "..name)
       dasm.dump(mcode, size)