x86: don't build unused entry code when !PV32

x86: don't build unused entry code when !PV32

Except for the initial part of cstar_enter compat/entry.S is all dead
code in this case. Further, along the lines of the PV conditionals we
already have in entry.S, make code PV32-conditional there too (to a
fair part because this code actually references compat/entry.S).

This has the side effect of moving the tail part (now at compat_syscall)
of the code out of .text.entry (in line with e.g. compat_sysenter).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wl@xen.org>