Add ARM HW feature detection module and add NEON detection

* configure.ac: Add option --disable-neon-support.
(HAVE_GCC_INLINE_ASM_NEON): New.
(ENABLE_NEON_SUPPORT): New.
[arm]: Add 'hwf-arm.lo' as HW feature module.
* src/Makefile.am: Add 'hwf-arm.c'.
* src/g10lib.h (HWF_ARM_NEON): New macro.
* src/global.c (hwflist): Add HWF_ARM_NEON entry.
* src/hwf-arm.c: New file.
* src/hwf-common.h (_gcry_hwf_detect_arm): New prototype.
* src/hwfeatures.c (_gcry_detect_hw_features) [HAVE_CPU_ARCH_ARM]: Add
call to _gcry_hwf_detect_arm.

Add HW detection module for detecting ARM NEON instruction set. ARM does not
have cpuid instruction so we have to rely on OS to pass feature set information
to user-space. For linux, NEON support can be detected by parsing
'/proc/self/auxv' for hardware capabilities information. For other OSes, NEON
can be detected by checking if platform/compiler only supports NEON capable
CPUs (by check if ARM_NEON macro is defined).

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