[REGRESSION] x86/entry: Tracer no longer has opportunity to change the syscall number at entry via orig_ax

[Kyle Huey]

tl;dr: after 27d6b4d14f5c3ab21c4aef87dd04055a2d7adf14 ptracer
modifications to orig_ax in a syscall entry trace stop are not honored
and this breaks our code.

rr, a userspace record and replay debugger[0], redirects syscalls of
its ptracee through an in-process LD_PRELOAD-injected solib. To do
this, it ptraces the tracee to a syscall entry event, and then, if the
syscall instruction is not our redirected syscall instruction, it
examines the tracee's code and pattern matches against a set of
syscall invocations that it knows how to rewrite. If that succeeds, rr
hijacks[1] the current syscall entry by setting orig_ax to something
innocuous like SYS_gettid, runs the hijacked syscall, and then
restores program state to before the syscall entry trace event and
allows the tracee to execute forwards, through the newly patched code
and into our injected solib.

Before 27d6b4d14f5c3ab21c4aef87dd04055a2d7adf14 modifications to
orig_ax were honored by x86's syscall_enter_trace[2]. The generic arch
code however does not honor any modifications to the syscall number[3]
(presumably because on most architectures syscall results clobber the
first argument and not the syscall number, so there is no equivalent
to orig_rax).

Note that the above is just one example of when rr changes the syscall
number this way. This is done in many places in our code and rr is
largely broken on 5.9-rc1 at the moment because of this bug.

- Kyle

[0] https://rr-project.org/
[1] https://github.com/mozilla/rr/blob/cd61ba22ccc05b426691312784674c0eb8e654ef/src/Task.cc#L872
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/entry/common.c?h=v5.8&id=bcf876870b95592b52519ed4aafcf9d95999bc9c#n204
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/entry/common.c?h=v5.8&id=27d6b4d14f5c3ab21c4aef87dd04055a2d7adf14#n44