Allow a test to raise privilege back to kernel level

Allow a test to raise privilege back to kernel level

to facilitate tests which switches privilege during the course of its run.

This is achieved by providing a dpl3 IDT entry which restores %esp and jumps
to the %eip found in the exception frame.

Therefore, 'int $0x20' acts as function call which returns at the kernels cpl.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>