x86: Control interrupts in vendor-specific way

Jailhouse runs with GIF cleared on AMD, and simple 'sti' and 'cli'
aren't enough to enable and disable interrupts. This affects apic_clear()
which fails to reset IRR on AMD. To overcome this, former enable_irq()
and disable_irq() are now defined in svm.c/vmx.c in vendor-specific way.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
[Jan: convert documentation into doxygen format]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
x86: Comments for apic_clear()

Added some comments for main steps that apic_clear() function
performs to restore APIC to the reset state.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
core: Explain asm-defines generation code origins

Explicitly mention code that was taken from Linux kernel in corresponding
source files' licensing headers.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>