Build issues in releng/11.0

[Ameya Deshpande.]

Hello to the mail recipients,

I am a student and new to FreeBSD Development. My work is specific to
FreeBSD 11.0-RELEASE. According to this document [1], branch
"releng/11.0" corresponds 11.0-RELEASE.

When I build the kernel by fetching "releng/11.0", I am getting some errors.

Initially, I got the error:

```
cc: error: unknown argument: '-fformat-extensions'
```

Unfortunately, I did not get a solution for this. But I found an
option in "tools/build/options" and tried building the kernel using
"make buildkernel WITHOUT_FORMAT_EXTENSIONS=yes".

This helped me to bypass the above error but I got another error:

```
In file included from /usr/src/sys/cam/cam.c:45:
In file included from /usr/src/sys/cam/cam_ccb.h:44:
In file included from /usr/src/sys/cam/nvme/nvme_all.h:32:
/usr/src/sys/dev/nvme/nvme.h:922:16: error: taking address of packed
member 'cdw10' of class or structure 'nvme_command' may result in an
      unaligned pointer value [-Werror,-Waddress-of-packed-member]
        *(uint64_t *)&cmd->cdw10 = lba;
                      ^~~~~~~~~~
1 error generated.
*** Error code 1
```

I am not facing any issues building other kernels versions like
'stable/11' or 'head'. Ideally, I assumed a similar build process for
'releng/11.0'. Please let me know how can I build a FreeBSD
11.0-RELEASE kernel.

Thanks,
Ameya Deshpande

[1]: https://www.freebsd.org/security/unsupported.html