UPSTREAM: crypto: blake2b - Fix clang optimization for ARMv7-M

When building for ARMv7-M, clang-9 or higher tries to unroll some loops,
which ends up confusing the register allocator to the point of generating
rather bad code and using more than the warning limit for stack frames:

warning: stack frame size of 1200 bytes in function 'blake2b_compress' [-Wframe-larger-than=]

Forcing it to not unroll the final loop avoids this problem.

Fixes: 91d689337fe8 ("crypto: blake2b - add blake2b generic implementation")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

(cherry picked from commit 0c0408e86dbe8f44d4b27bf42130e8ac905361d6)
Bug: 178411248
Change-Id: I71ee6df844bdbace9f06342a61322c7494533564
Signed-off-by: Eric Biggers <ebiggers@google.com>