This allows the static linker to drop it in consumers which don't need this
stuff (i.e. all sane ones), once crypto/x509 falls off. This cuts down
on a number of dependencies from the core crypto bits on crypto/asn1 and
crypto/x509.
BUG=499653
Change-Id: I76a10a04dcc444c1ded31683df9f87725a95a4e6
Reviewed-on: https://boringssl-review.googlesource.com/5660
Reviewed-by: Adam Langley <agl@google.com>