Add a tool for patching ELF libraries/executables offline

Add a tool for patching ELF libraries/executables offline

This is for patching ELF objects for use with elf_loader.cc.  It
allows system call instructions to be patched before the library or
executable is loaded.

For simplicity, we replace system calls with "int $0", which is not
very fast at run time.  A more sophisticated version could insert
jumps like library.cc does, but this would involve adding an extra
code segment to the ELF object.

BUG=http://code.google.com/p/seccompsandbox/issues/detail?id=13
TEST=For example:
  ./patch_offline /lib/ld-linux.so.2 -o ld.so.patched
  objdump -d ld.so.patched | grep -w int
Automated tests will be added in a later change.

Committed: http://code.google.com/p/seccompsandbox/source/detail?r=173

[Markus (顧孟勤), 2011-11-17 22:15:19]

lgtm

http://codereview.chromium.org/8589027/diff/2001/patch_offline.cc
File patch_offline.cc (right):

http://codereview.chromium.org/8589027/diff/2001/patch_offline.cc#newcode15
patch_offline.cc:15: // that they will work inside the sandbox.
I suspect, in the long run, you probably want a tool that can do this at
run-time. In that case, you also wouldn't be limited to inserting "int $0"
instructions.

But for now, this seems perfectly fine.