Use arm64 sha256 intrinsics in libmd

[andrew]

Summary

When running on a CPU that supports the arm64 sha256 intrinsics use them
to improve perfromance of sha256 calculations.

With this changethe following improvement has been seen on an Apple M1
with FreeBS running under Parallels, with similar results on a
Neoverse-N1 r3p1.

x sha256.orig
+ sha256.arm64
+--------------------------------------------------------------------+

++ x x
+++ xxx
	A	A

+--------------------------------------------------------------------+

N           Min           Max        Median           Avg        Stddev

x 5 3.41 3.5 3.46 3.458 0.042661458
+ 5 0.47 0.54 0.5 0.504 0.027018512
Difference at 95.0% confidence

-2.954 +/- 0.0520768
-85.4251% +/- 0.826831%
(Student's t, pooled s = 0.0357071)

[andrew]

Phabricator doesn't like the ministat plot so I've copied it here:

x sha256.orig
+ sha256.arm64
+--------------------------------------------------------------------+
|++                                                               x x|
|+++                                                              xxx|
||A                                                               |A||
+--------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   5          3.41           3.5          3.46         3.458   0.042661458
+   5          0.47          0.54           0.5         0.504   0.027018512
Difference at 95.0% confidence
        -2.954 +/- 0.0520768
        -85.4251% +/- 0.826831%
        (Student's t, pooled s = 0.0357071)

[cem]

sys/crypto/sha2/sha256c.c

198

I might throw this in a header file.

201

Static?

[andrew]

Add static

[andrew]

sys/crypto/sha2/sha256c.c

198

The only reason I didn't is because I didn't want to add it to sha256.h as it shouldn't be used outside of this code and I didn't want to create a new header just for 2 lines.

[andrew]

Create a new header for the SHA256_Transform_arm64_impl signature