Add support for using DRNG random number generator

* configure.ac: Add option --disable-drng-support.
(ENABLE_DRNG_SUPPORT): New.
* random/rndhw.c (USE_DRNG): New.
(rdrand_long, rdrand_nlong, poll_drng): New.
(_gcry_rndhw_poll_fast, _gcry_rndhw_poll_slow): Call poll function.
* src/g10lib.h (HWF_INTEL_RDRAND): New.
* src/global.c (hwflist): Add "intel-rdrand".
* src/hwfeatures.c (detect_x86_64_gnuc) [ENABLE_DRNG_SUPPORT]: Detect
RDRAND.
(detect_ia32_gnuc) [ENABLE_DRNG_SUPPORT]: Detect RDRAND.

This patch provides support for using Digital Random Number Generator (DRNG)
engine, which is available on the latest Intel's CPUs. DRNG engine is
accesible via new the RDRAND instruction.

This patch adds the following:

• support for disabling using of rdrand instruction
• checking for RDRAND instruction support using cpuid
• RDRAND usage implementation

ChangeLog and editorial changes by wk.

• Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>