Instead of copying data into and out of a fake CONTEXT structure with
only a few entries, use the platform specific structure directly with a
typedef and macros. This is needed because fastmem writes need to be
able to access any register from BackPatch. It adds a fair number of
repetitive defines, but it's better than the alternative.
It isn't easily accessible with sigaction or Mach exceptions (well,
requires an additional system call in the latter), and isn't necessary.
(and get rid of the enum, because it's only used once, and the comments
are more expressive than enum names)
