[dev.boringcrypto] api: add crypto/boring.Enabled

Change-Id: Ifdb40b5df9ab435e1dd585dc21fd4a93fad9b5cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/296749
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

api/go1.16.txt

@@ -1,5 +1,6 @@
 pkg archive/zip, method (*ReadCloser) Open(string) (fs.File, error)
 pkg archive/zip, method (*Reader) Open(string) (fs.File, error)
+pkg crypto/boring, func Enabled() bool
 pkg crypto/x509, method (SystemRootsError) Unwrap() error
 pkg debug/elf, const DT_ADDRRNGHI = 1879047935
 pkg debug/elf, const DT_ADDRRNGHI DynTag