[PATCH] Remove incorrect default implementation of atomics.

[Roland McGrath]

My point is that removing the header entirely is not good.  When generic
code requires that some sysdeps file exist, then we always have a generic
or stub version of that sysdeps file that tells someone about how to go
about filling in that requirement in a new port.  If there is really
nothing useful for the placeholder file to contain, it can contain an
#error line and a comment referring to where the relevant details are to be
found.  But in most cases, we have found it most useful to write a skeleton
file (maybe still with some #error or other obviously non-compiling syntax)
that has at least substantial comments about what a real file would do.