Work around Erratum 721 for AMD Family 10h and 12h processors.
Work around Erratum 721 for AMD Family 10h and 12h processors.

"Under a highly specific and detailed set of internal timing conditions,
the processor may incorrectly update the stack pointer after a long series
of push and/or near-call instructions, or a long series of pop and/or
near-return instructions.  The processor must be in 64-bit mode for this
erratum to occur."

MFC after:	3 days

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