Cache invalidated might fail on Mac M1/M2

[pgarba]

The cache invalidation might fail if a new function is allocated at the same address as the cached one.
This happens to me on the M2 after I delete a function and then allocate a new one which leads to return a nullptr for cached values and results in a crash.

remill/lib/BC/InstructionLifter.cpp

Line 96 in 829ee4a

if (func != impl->last_func) {

[pgoodman]

Perhaps the cache invalidation should hold a weak value handle on the llvm function.