Bump CSTD to gnu11 for world and kernel.

Bump CSTD to gnu11 for world and kernel.

In practice, I don't think this will change a lot for base code, and I
am doing it mainly for contrib/ code which might in certain cases use
different paths (for example zstd will). Also affected might be dports
which use base's share/mk infrastructure, of which there are a few.

I have compared the object directories with all temporary files (.s, .i)
between a LINT64 kernel compiled with -std=gnu11 vs. one built with c99
and there were no differences whatsoever.

Thanks to tuxillo for testing with a full bulk build.

Note that the kernel was previously using c99 and has now been switched
to gnu11, not c11. This is because we were always using GNU extensions
in kernel code, so c99 should really have been gnu99 always.

Also note that we could in theory have switched to gnu17 directly since
c17 only addressed some defects in c11 and did not add new features,
but I chose to use 11 because our secondary compiler does not support
17.

Reminded-by: Peeter Must