Optimize _gcry_burn_stack for 32-bit and 64-bit architectures

* src/misc.c (_gcry_burn_stack): Add optimization for 32-bit and 64-bit
architectures.

Busy looping 'tests/benchmark --cipher-repetitions 10 cipher blowfish' on ARM
Cortex-A8 shows that _gcry_burn_stack takes 21% of CPU time. With this patch,
that number drops to 3.4%.

On AMD64 (Intel i5-4570) CPU usage for _gcry_burn_stack in the same test drops
from 3.5% to 1.1%.

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