arm64: Don't enable interrupts when in a spinlock

[andrew]

Summary

When we receive an exception while in a spinlock we shouldn't enable
interrupts. When entering a spinlock we disable interrupts so enabling
them here could cause surprising results.

The three cases that could cause this are:

1. A break-before-make sequence
2. Accessing possibly unmapped code with a fault handler
3. Buggy code

1 and 2 are supported later in the data abort handler, and 3 should be
fixed when found.

Sponsored by: Arm Ltd