Rebuilt the patch-sigev using more coherent names and redefining SIGLIBRT

This patch could be pushed to gdb upstream
Adding --with-separate-debug-dir pointing to /usr/lib/debug, because FreeBSD install separate debug symbol files there

The previous default value was /usr/local/lib/debug that was unused.
This patch follows a request of Andriy Gapon (avg@)
Updating to gdb 7.6.2