rijndael: add ARMv6 assembly implementation
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rijndael: add ARMv6 assembly implementation

* cipher/Makefile.am: Add 'rijndael-armv6.S'.
* cipher/rijndael-armv6.S: New file.
* cipher/rijndael.c (USE_ARMV6_ASM): New macro.
[USE_ARMV6_ASM] (_gcry_aes_armv6_encrypt_block)
(_gcry_aes_armv6_decrypt_block): New prototypes.
(do_encrypt_aligned) [USE_ARMV6_ASM]: Use ARMv6 assembly function.
(do_encrypt): Disable input/output alignment when USE_ARMV6_ASM.
(do_decrypt_aligned) [USE_ARMV6_ASM]: Use ARMv6 assembly function.
(do_decrypt): Disable input/output alignment when USE_ARMV6_ASM.
* configure.ac (HAVE_COMPATIBLE_GCC_ARM_PLATFORM_AS): New check for
gcc/as compatibility with ARM assembly implementations.
(aes) [arm]: Add 'rijndael-armv6.lo'.

Add optimized ARMv6 assembly implementation for AES. Implementation is tuned
for Cortex-A8. Unaligned access handling is done in assembly part.

For now, only enable this on little-endian systems as big-endian correctness
have not been tested yet.

Old vs new. Cortex-A8 (on Debian Wheezy/armhf):

   ECB/Stream         CBC             CFB             OFB             CTR
--------------- --------------- --------------- --------------- ---------------

AES 2.61x 3.12x 2.16x 2.59x 2.26x 2.25x 2.08x 2.08x 2.23x 2.23x
AES192 2.60x 3.06x 2.18x 2.65x 2.29x 2.29x 2.12x 2.12x 2.25x 2.27x
AES256 2.62x 3.09x 2.24x 2.72x 2.30x 2.34x 2.17x 2.19x 2.32x 2.32x