This commit introduces the association of GOT indices to the LLVM index,
which then allows us to utilise that when generating references to
variables to check if a given variable has an entry in the GOT. If so,
we obtain its index, and generate the necessary LLVM IR to access the
address contained within the GOT rather than accessing the variable
directly.

This change involves moving the generation of the GOT from
variable_generator.rs to codegen.rs, in order to also cover not only
global variables but also functions and 'programs' too. Once these have
been given an associated index in the GOT we can use that to replace
normal direct function calls with indirect calls to a function pointer
stored in the GOT. We don't do this for calls with external linkage
since these won't be subject to online change.