Do CI compile with xgo

This will hopefully fix Travis errors:

/home/travis/.gimme/versions/go1.11.7.linux.amd64/pkg/tool/linux_amd64/link: running gcc failed: exit status 1
/usr/bin/ld: /tmp/go-link-810454258/000020.o: unrecognized relocation (0x2a) in section `.text'
/usr/bin/ld: final link failed: Bad value