arm64: explicitly mask all exceptions

There are a few places where we want to mask all exceptions. Today we
do this in a piecemeal fashion, typically we expect the caller to
have masked irqs and the arch code masks debug exceptions, ignoring
serror which is probably masked.

Make it clear that 'mask all exceptions' is the intention by adding
helpers to do exactly that.

The caller should update trace_hardirqs where appropriate, adding this
logic to the mask/unmask helpers causes asm/irqflags.h and
linux/irqflag.h to include each other.

Signed-off-by: James Morse <james.morse@arm.com>