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: 91d68933 ("crypto: blake2b - add blake2b generic implementation")
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Reviewed-by: NNathan Chancellor <natechancellor@gmail.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

crypto/blake2b_generic.c

@@ -129,7 +129,9 @@ static void blake2b_compress(struct blake2b_state *S,
 	ROUND(9);
 	ROUND(10);
 	ROUND(11);
-
+#ifdef CONFIG_CC_IS_CLANG
+#pragma nounroll /* https://bugs.llvm.org/show_bug.cgi?id=45803 */
+#endif
 	for (i = 0; i < 8; ++i)
 		S->h[i] = S->h[i] ^ v[i] ^ v[i + 8];
 }