Remove sub-arch/variants selection from the menuconfig

[Yann E. MORIN]

It is not the responsibility of the C library to select what sub-arch
to generate code for. Rely on the user to:
- either use a compiler that is configured to generate optimised code
  for the target actual target,
- or set UCLIBC_EXTRA_CFLAGS with the correct incantation of -march,
  -mcpu, and the likes.