Don't export __chown

[Roland McGrath]

> I think it is an oversight that __chown@GLIBC_2.1 is exported on i386.

I agree.  This symbol was never in a Versions file.  Probably noone
realized that .symver definitions would result in exported symbols that
aren't listed in any Versions file (i.e. in the libc.map built from them)
and people only ever did visual checks on the Versions files.  Score
another one for check-abi.

I am inclined to change the generation of .map files to put "local: *;" at
the end of each set so that nothing not listed in a Versions file goes
through.