Add an arm64 sha512 feature check

This is only available in recent toolchains. Check for it so we can
use it in libmd.

Reviewed by:	emaste
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33166

share/mk/bsd.compiler.mk

@@ -25,6 +25,7 @@
 # - retpoline: supports the retpoline speculative execution vulnerability
 #              mitigation.
 # - init-all:  supports stack variable initialization.
+# - aarch64-sha512: supports the AArch64 sha512 intrinsic functions.
 #
 # When bootstrapping on macOS, 'apple-clang' will be set in COMPILER_FEATURES
 # to differentiate Apple's version of Clang. Apple Clang uses a different
@@ -255,6 +256,13 @@ ${X_}COMPILER_FEATURES+=	compressed-debug
 ${X_}COMPILER_FEATURES+=	fileprefixmap
 .endif
 
+.if (${${X_}COMPILER_TYPE} == "clang" && ${${X_}COMPILER_VERSION} >= 130000) || \
+	(${${X_}COMPILER_TYPE} == "gcc" && ${${X_}COMPILER_VERSION} >= 90000)
+# AArch64 sha512 intrinsics are supported (and have been tested) in
+# clang 13 and gcc 9.
+${X_}COMPILER_FEATURES+=	aarch64-sha512
+.endif
+
 .else
 # Use CC's values
 X_COMPILER_TYPE=	${COMPILER_TYPE}