x86/entry: Use generic syscall exit functionality

Replace the x86 variant with the generic version. Provide the relevant
architecture specific helper functions and defines.

Use a temporary define for idtentry_exit_user which will be cleaned up
seperately.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20200722220520.494648601@linutronix.de