Improved structure of LDT-related code:

Improved structure of LDT-related code:
- one declarations from core.h removed, one moved to within m_syscalls.
- all the x86 LDT stuff made local to m_syscalls.  x86-linux/ldt.c removed
  as a result.  x86/state.c slimmed down, too.  x86/x86_private.h removed
  too.
- all the AMD64 LDT stuff was deleted, since it was all commented out.  It
  can be added back in later in the appropriate places if necessary.
  Thus amd64-linux/ldt.c and amd64/amd64_private.h were removed.
- other minor naming changes

I hope I didn't break AMD64 compilation.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@3726