BACKPORT: x86/entry: Get rid of two-phase syscall entry work

I added two-phase syscall entry work back when the entry slow path
was very slow.  Nowadays, the entry slow path is fast and two-phase
entry work serves no purpose.  Remove it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
(cherry picked from commit c87a85177e7a7f9a9ee32893fb99a928e02fe23a)

Bug: 119769499
Change-Id: Ieac4470411f88ca8830794d0322d8d8bb348039e
[ghackmann@google.com:
 - adjust for post-4.4 is_ia32_task() -> in_ia32_syscall() renaming
 - preserve TF flags fixup in syscall_trace_enter()
 - keep syscall_trace_enter() exported, since we haven't taken
   patches to move the calling code from entry_64.S to common.c]
Signed-off-by: Greg Hackmann <ghackmann@google.com>