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

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

fix ABI detection logic: for 64-bit elf objects set default ABI to n64.

Calculate offset from frame top for registers saved on a stack frame.

This commit was generated by cvs2svn to compensate for changes in r130803,
which included commits to RCS files with non-trunk default branches.

Import of GDB version 6.1.1 (stripped; see FREEBSD-Xlist rev. 1.6).