runtime: optimize zeroing of registers in secret_amd64.s

[CAFxX]

Use VPXORQ instead of VMOVAPD because the former, when in the form of a zeroing idiom, is handled directly by the renamer.

Tweak also the KXORQs to operate each on a single register, making it trivial to understand what the intent is, and so that all can potentially execute in parallel.

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Message from Gopher Robot:

Patch Set 1:

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Patch Set 1:

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Message from Carlo Alberto Ferraris:

Patch Set 2:

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Message from Carlo Alberto Ferraris:

Patch Set 2:

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