use curl with OpenSSL backend on 16.04-cross based containers

[wfurt]

This contributes to #34015 This pulls in containers built with dotnet/dotnet-buildtools-prereqs-docker#295 As GnuTLS being suspect, this will switch to OpenSSL backend for curl.

furt@localhost:~$ docker run mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-cross-20200413125008-09ec757 curl --version
curl 7.47.0 (x86_64-pc-linux-gnu) libcurl/7.47.0 OpenSSL/1.0.2g zlib/1.2.8 libidn/1.32 librtmp/2.3
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp smb smbs smtp smtps telnet tftp
Features: AsynchDNS IDN IPv6 Largefile GSS-API Kerberos SPNEGO NTLM NTLM_WB SSL libz TLS-SRP UnixSockets

furt@localhost:~$ docker run mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-cross-arm64-20200413125008-cfdd435  curl --version
curl 7.47.0 (x86_64-pc-linux-gnu) libcurl/7.47.0 OpenSSL/1.0.2g zlib/1.2.8 libidn/1.32 librtmp/2.3
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp smb smbs smtp smtps telnet tftp
Features: AsynchDNS IDN IPv6 Largefile GSS-API Kerberos SPNEGO NTLM NTLM_WB SSL libz TLS-SRP UnixSockets

furt@localhost:~$ docker run mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-cross-arm64-alpine-20200413125008-406629a   curl --version
curl 7.47.0 (x86_64-pc-linux-gnu) libcurl/7.47.0 OpenSSL/1.0.2g zlib/1.2.8 libidn/1.32 librtmp/2.3
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp smb smbs smtp smtps telnet tftp
Features: AsynchDNS IDN IPv6 Largefile GSS-API Kerberos SPNEGO NTLM NTLM_WB SSL libz TLS-SRP UnixSockets

[ghost]

Tagging @ViktorHofer as an area owner.

[safern]

Alpine was also showing in the failures right? Is there something we're going to do there?

[wfurt]

my thinking was to get the 16.04-cross in place and see if that makes difference (hopefully positive) If we can blame GnuTLS, I can work out remaining build queues.

[jashook]

Can you also update the documentation?

[wfurt]

Can you also update the documentation?

documentaton where?

[jashook]

https://github.com/dotnet/runtime/blob/master/docs/workflow/building/coreclr/linux-instructions.md#Docker-Images