x86emul: fix SYSENTER/SYSCALL switching into 64-bit mode

x86emul: fix SYSENTER/SYSCALL switching into 64-bit mode

When invoked by compat mode, mode_64bit() will be false at the start of
emulation. The logic after complete_insn, however, needs to consider the
mode switched into, in particular to avoid truncating RIP.

Inspired by / paralleling and extending Linux commit 943dea8af21b ("KVM:
x86: Update emulator context mode if SYSENTER xfers to 64-bit mode").

While there, tighten a related assertion in x86_emulate_wrapper() - we
want to be sure to not switch into an impossible mode when the code gets
built for 32-bit only (as is possible for the test harness).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citirix.com>