kernel/x86_64: add a compatibility syscalls table.

* install syscall handlers in compatibility mode.
* copy the syscall entry in the compatibility commpage, ATM only for Intel
SYSENTER.
* copy the t

kernel/x86_64: add a compatibility syscalls table.

* install syscall handlers in compatibility mode.
* copy the syscall entry in the compatibility commpage, ATM only for Intel
SYSENTER.
* copy the thread exit handler in the compatibility commpage.

Change-Id: Ic350799938815194377d8a4560cb106fe7366cc6

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