"ldr <reg>, <literal>" loads a value from a literal memory address into
a register. It's PC-relative and so cannot be directly implemented
using the trampoline mechanism. Unfortunately, on arm64 it can't easily
be emulated either since the return-to-EL1 handler does not restore
callee-saved registers, so like adr/adrp, we simply don't handle it.
These instructions are fairly rare in an arm64 kernel.
While here, refactor the code so that all instruction decoding is done
in one place: introduce an enum type which characterizes the instruction
type, add a helper to map instructions to enum values, and store the
corresponding enum value in the probe description.