boring: update documentation to include arm64

Support for boring has been extended to include linux/arm64. This change
updates the docs to reflect that.

Fixes #63920

Change-Id: If8d6eca713e8245dcc222c3e38d140874d48725d
Reviewed-on: https://go-review.googlesource.com/c/go/+/539298
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/crypto/internal/boring/doc.go

@@ -10,7 +10,7 @@ package boring
 // Enabled reports whether BoringCrypto is available.
 // When enabled is false, all functions in this package panic.
 //
-// BoringCrypto is only available on linux/amd64 systems.
+// BoringCrypto is only available on linux/amd64 and linux/arm64 systems.
 const Enabled = available
 
 // A BigInt is the raw words from a BigInt.