x86/dumpstack: Use uniform "Oops: " prefix for die() messages

panic() prints a uniform prompt: "Kernel panic - not syncing:",
but die() messages don't have any of that, the message is the
raw user-defined message with no prefix.

There's companies that collect thousands of die() messages per week,
but w/o a prompt in dmesg, it's hard to write scripts to collect and
analize the reasons.

Add a uniform "Oops:" prefix like other architectures.

[ mingo: Rewrote changelog. ]

Signed-off-by: Alex Shi <alexs@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240327024419.471433-1-alexs@kernel.org