[dev.power64] cmd/5a, cmd/6a, cmd/8a, cmd/9a: make labels function-scoped

I removed support for jumping between functions years ago,
as part of doing the instruction layout for each function separately.

Given that, it makes sense to treat labels as function-scoped.
This lets each function have its own 'loop' label, for example.

Makes the assembly much cleaner and removes the last
reason anyone would reach for the 123(PC) form instead.

Note that this is on the dev.power64 branch, but it changes all
the assemblers. The change will ship in Go 1.5 (perhaps after
being ported into the new assembler).

Came up as part of CL 167730043.

LGTM=r
R=r
CC=austin, dave, golang-codereviews, minux
https://golang.org/cl/159670043

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