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

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

Compile fbsd-threads.c. Amongst others this is necessary for a working
kgdb(1).

Reviewed by: marcel
Approved by: re (kib)
MFC after: 1 week

Source changes to allow building a cross-debugger. Move solib.c and
solib-svr4.c to the MD makefiles because they are native files for
alpha and sparc64, but target files for amd64, i386 and ia64.

Note that kgdb(1) does not yet build as a cross-debugger due to
libkvm.

Add bmake glue for gdb 6.1.1. Supports alpha, amd64, i386, ia64
and sparc64.

Note that the debugger doesn't support threading, nor kernel
debugging yet.