Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Export quad symbols. They were previously commented out. These symbols
really shouldn't be exported since they should be pulled from libgcc, but
the build of some applications is broken and they expect to see them in
libc. glibc exports these symbols, although Solaris doesn't appear to,
so export them for compatibility's sake.

After discussion with: kan

Use C comments since we now preprocess these files with CPP.

Add symbol maps and initial symbol version definitions to libc.

Reviewed by: davidxu