target-arm: add support for v8 SHA1 and SHA256 instructions

This adds support for the SHA1 and SHA256 instructions that are available
on some v8 implementations of Aarch32.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1401386724-26529-2-git-send-email-peter.maydell@linaro.org
[PMM:
 * rebase
 * fix bad indent
 * add a missing UNDEF check for Q!=1 in the 3-reg SHA1/SHA256 case
 * use g_assert_not_reached()
 * don't re-extract bit 6 for the 2-reg-misc encodings
 * set the ELF HWCAP2 bits for the new features
]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
