gccgo: add AIX support

[MatthieuSarter]

What version of Go are you using (go version)?

Snapshots from gcc SVN repository

What operating system and processor architecture are you using (go env)?

AIX 6.1/7.2 on Power7/8

What did you do?

We are currently porting gccgo to AIX. This issue will be used to submit and discuss our changes.

[gopherbot]

CL https://golang.org/cl/37633 mentions this issue.

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CL https://golang.org/cl/37637 mentions this issue.

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CL https://golang.org/cl/37638 mentions this issue.

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CL https://golang.org/cl/37632 mentions this issue.

[dimitertodorov]

How ready is this port for general testing.
We have to build some agents for our AIX environment, and GO seems like a good candidate.

D

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CL https://golang.org/cl/37842 mentions this issue.

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CL https://golang.org/cl/37844 mentions this issue.

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CL https://golang.org/cl/37845 mentions this issue.

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CL https://golang.org/cl/37846 mentions this issue.