Fix a regression in r338360 when booting an x86 machine without APIC.

The atpic_register_sources callback tries to avoid registering interrupt
sources that would collide with an I/O APIC. However, the previous
implementation was failing to register IRQs 8-15 since the slave PIC
saw valid IRQs from the master and assumed an I/O APIC was present. To
fix, go back to registering all 8259A interrupt sources in one loop when
the master's register_sources method is invoked.

PR: 231291
Approved by: re (kib)
MFC after: 1 month