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.
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