Add volatile qualifications to two blocks of inline asm to stop GCC from
eliminating them as dead code.

Both volatile and "memory" are used because of some concern that the compiler
may still cache values across the asm block without it, and because this was
such a painful debugging session that I wanted to ensure that it's never
repeated.
Ensure that x**0 mod 1 = 0.
Ensure that, when generating small primes, the result is actually of the
requested size. Fixes OpenSSL #2701.

This change does not address the cases of generating safe primes, or
where the |add| parameter is non-NULL.

Conflicts:
	crypto/bn/bn.h
	crypto/bn/bn_err.c