Add in exec_context.hh, which is a file for documentation purposes only. It describe...

Add in exec_context.hh, which is a file for documentation purposes only.  It describes the ExecContext interface that the ISA uses to access CPU state.  Also #ifdef Erik's old copy code from the decoder so ExecContext doesn't need his two specific copy functions.

src/arch/alpha/isa/decoder.isa:
    Surround Erik's old copy code with #ifdefs.  This way the copy functions don't need to be included in the ExecContext (until somebody decides to add them back in).

--HG--
extra : convert_revision : 508ca387757a32bb616e5b4b07af17787a76970e