Parse /proc/cpuinfo for ARM HW features

* src/hwf-arm.c [__linux__] (HAS_PROC_CPUINFO)
(detect_arm_proc_cpuinfo): New.
(_gcry_hwf_detect_arm) [HAS_PROC_CPUINFO]: Check '/proc/cpuinfo' for
HW features.

Some Linux platforms (read: Android) block read access to '/proc/self/auxv',
which prevents NEON HW detection. Patch adds alternative check which parses
'/proc/cpuinfo' which should be accessable by Android applications.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>