x86/xen: Rename SYS* entry points

Native SYS{CALL,ENTER} entry points are called
entry_SYS{CALL,ENTER}_{64,compat}, make sure the Xen versions are
named consistently.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>