[ARM] bits/hwcap.h update

[Szabolcs Nagy]

Since https://www.sourceware.org/ml/libc-alpha/2014-06/msg00756.html
the AT_HWCAP2 bits are handled on arm so the linux uapi macros should
be defined if sys/auxv.h is included.

I can propose a patch, however it seems the glibc names for HWCAP
bits are different than the linux names: HWCAP_ARM_.. vs HWCAP_..

These are linux specific flags as noted in
https://sourceware.org/ml/libc-ports/2012-08/msg00069.html
so i'm not sure why the difference.

Does anybody know the reason?

(I'd prefer adding all the current linux uapi names to glibc
and keeping the HWCAP_ARM_ macros for compatibility).