Fix building AArch32 CE implementations when target is ARMv6 arch

* cipher/cipher-gcm-armv8-aarch32-ce.S: Select ARMv8 architecure.
* cipher/rijndael-armv8-aarch32-ce.S: Ditto.
* cipher/sha1-armv8-aarch32-ce.S: Ditto.
* cipher/sha256-armv8-aarch32-ce.S: Ditto.
* configure.ac (gcry_cv_gcc_inline_asm_aarch32_crypto): Ditto.

Raspbian distribution defaults to ARMv6 architecture thus 'rbit'
instruction is not available with default compiler flags. Patch
adds explicit architecture selection for ARMv8 to enable 'rbit'
usage with ARMv8/AArch32-CE assembly implementations of SHA,
GHASH and AES.

• Reported-by: Chris Horry <zerbey@gmail.com>
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>