linux(4): fix ptrace(2) to properly handle orig_rax

[trasz]

Summary

This should fix strace(1) erroneously reporting return values
as "Function not implemented", combined with reporting the binary
ABI as X32.

Very similar code in linux_ptrace_getregs() is left as it is - it might
be wrong, but I don't have a way to test it.

[jhb]

In general Linux extends GPR regsets with 'orig_foo' that stores the syscall code. Presumably i386 should be doing the same for orig_eax?

[trasz]

In D29927#671791, @jhb wrote:

In general Linux extends GPR regsets with 'orig_foo' that stores the syscall code. Presumably i386 should be doing the same for orig_eax?

Perhaps, but I don't have an easy way to test i386 either.

[trasz]

(Tinderboxed.)