Introduce a spinlock for synchronizing access to the video output hardware

in syscons.  This replaces a simple access semaphore that was assumed to be
protected by Giant but often was not.  If two threads that were otherwise
SMP-safe called printf at the same time, there was a high likelyhood that
the semaphore would get corrupted and result in a permanently frozen video
console.  This is similar to what is already done in the serial console
drivers.

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