arch/arm: remove OABI option

OABI is more than legacy, it's dead.

New developments should go with EABI, since it so much better.
>From the Debian EABI page [0] :
  - floating point performance, with or without an FPU is very much faster
  - mixing soft and hardfloat code is possible
  - structure packing is not as painful as it used to be
  - a more efficient syscall convention
  - more compatibility with various tools

[0] http://wiki.debian.org/ArmEabiPort



[Thomas: keep the ABI choice, as we are going to introduce EABIhf later].
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>