x86_64: Add endbranch at function entries for Intel CET

x86_64: Add endbranch at function entries for Intel CET

To support Intel CET, all indirect branch targets must start with
endbranch.  Here is a patch to add endbranch to function entries
in x86_64 assembly codes which are indirect branch targets as
discovered by running openssl testsuite on Intel CET machine and
visual inspection.

Verified with

$ CC="gcc -Wl,-z,cet-report=error" ./Configure shared linux-x86_64 -fcf-protection
$ make
$ make test

and

$ CC="gcc -mx32 -Wl,-z,cet-report=error" ./Configure shared linux-x32 -fcf-protection
$ make
$ make test # <<< passed with https://github.com/openssl/openssl/pull/10988

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10982)