Create dummy target arch for tests

[codyroux]

We currently use the assumption we're on X86_64 pervasively and scattered throughout in the tests, eg.

cbat_tools/wp/lib/bap_wp/tests/unit/test_utils.ml

Line 36 in 050d8bc

let x86_64_input_regs : Var.t list =

cbat_tools/wp/lib/bap_wp/tests/unit/test_utils.ml

Line 128 in 050d8bc

let test_tgt = X86_target.amd64

cbat_tools/wp/lib/bap_wp/tests/unit/test_compare.ml

Line 655 in 050d8bc

let module Target = (val target_of_arch `x86_64) in

cbat_tools/wp/lib/bap_wp/tests/unit/test_compare.ml

Line 696 in 050d8bc

let module Target = (val target_of_arch `x86_64) in

We need to:

• Make things generic over the target
• Create our own dummy target with the appropriate variables for the needed roles in the analysis.
• If there are x86 or AMD64 specific tests, make sure to mark them appropriately.