'RELENG_5_3_0_RELEASE'.

This commit was manufactured to restore the state of the 5.3-RELEASE image.

Spotted by:	tmm

  same size.  Add some fields that previously overlapped with something else
  or were missing.
- Make struct regs and struct mcontext (minus floating point) the same as
  struct trapframe so converting between them is easy (null).
- Add space for saving floating point state to struct mcontext.  This requires
  that it be 64 byte aligned.
- Add assertions that none of these structures change size, as they are part
  of the ABI.
- Remove some dead code in sendsig().
- Save and restore %gsr in struct trapframe.  Remember to restore %fsr.
- Add some comments to exception.S.

2. Remove mcontext.mc_sp, it is redundant.  Adjust spare space to make
   ucontext_t a nice size.
3. Raise pil in the debugger.

Submitted by:	tmm (3)

to a new architecture.  This is the base of the sparc64 port, but contains
limited machine dependent code, and can be used a base for ports.  Included
are:
- standard machine dependent headers, tweaked for a 64 bit, big endian
  architecture, including empty versions of all the machine dependent
  structures
- a machine independent atomic.h, which can be used until a port has
  support for interrupts and the operations really need to be atomic
- stub versions of all the machine dependent functions, which panic
  when called and print out the name of the function that needs to
  be implemented.  functions which are normally in assembly files are
  not included, but this should reduce the number of different undefined
  references on the first few compiles from hundreds to 5 or 6
Given minimal startup code and console support it should be trivial to
make this compile and run the first few sysinits on almost any architecture.

Requested by:   alfred, imp, jhb