arm64: entry: improve bad_mode()

Our use of bad_mode() has a few rough edges:

* AArch64 doesn't use the term "mode", and refers to "Execution
  states", "Exception levels", and "Selected stack pointer".

* We log the exception type (SYNC/IRQ/FIQ/SError), but not the actual
  "mode" (though this can be decoded from the SPSR value).

* We use bad_mode() as a second-level handler for unexpected synchronous
  exceptions, where the "mode" is legitimate, but the specific exception
  is not.

* We dump the ESR value, but call this "code", and so it's not clear to
  all readers that this is the ESR.

... and all of this can be somewhat opaque to those who aren't extremely
familiar with the code.

Let's make this a bit clearer by having bad_mode() log "Unhandled
${TYPE} exception" rather than "Bad mode in ${TYPE} handler", using
"ESR" rather than "code", and having the final panic() log "Unhandled
exception" rather than "Bad mode".

In future we'd like to log the specific architectural vector rather than
just the type of exception, so we also split the core of bad_mode() out
into a helper called __panic_unhandled(), which takes the vector as a
string argument.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210607094624.34689-13-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

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