[dev.power64] cmd/9c: fix constant truncation in optimizer

This is what broke getpopcount, but that code had it coming.

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https://codereview.appspot.com/130000044
[dev.power64] runtime: replace getproccount with simpler code

This runs once. There is no need for inscrutable algorithms.
Also it doesn't compile correctly with 9c.

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https://codereview.appspot.com/130000043
[dev.power64] cmd/9g: correct floating point comparison

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https://codereview.appspot.com/129210043
[dev.power64] cmd/dist: tell goc2c that power64 is 64-bit system

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https://codereview.appspot.com/125300043
[dev.power64] reflect: fix test not to hang even if TrySend lies

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https://codereview.appspot.com/124360043