dwarf: fixup x86 register mapping logic
dwarf: change the register context update to update in-place instead of copying
debug: always print the unwind error type
Move to c/darwin.zig as they really are libSystem/libc imports/wrappers.
As an added bonus, get rid of the nasty `usingnamespace`s which are now
unneeded.
Finally, add `os.ptrace` but currently only implemented on darwin.
