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

This fixes 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's
probably wrong too, but I don't have a way to test it.

Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D29927