Use arm64 sha256 intrinsics in libmd

Use arm64 sha256 intrinsics in libmd

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)

Reviewed by:	cem
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31284