CrypoAlg-556: Enable tests on assembly functions

- As per Torben's comments, the tests in `p256-x86_64_test.cc` for the assembly
functions were only enabled for x86_64. In this change, they are enabled
for ARMv8 assembly as well, excluding the functions that are to be implemented.
- Other comments from Torben were also addressed:
  - cleaning up TODOs, globals, #ifdefs
  - removing the gather/scatter routines from `p256-armv8-asm.pl`
    and the corresponding (what I believe is) scattering of the table
    of precomputed points.
- Fixed a typo in a comment in `ecp_nistz256_select_w5`.

Tested on:
- ARMv8 (M6g)
- x86_64 (MacBook)