gccgo: does not build on Darwin

[wrabbit-revisited]

Tried building via the procedure documented at https://golang.org/doc/install/gccgo using the gccgo branch. Hit a problem during configure. iant on the IRC informs me that Darwin is currently not supported. Not a blocker, but based on https://sourceware.org/gdb/wiki/GoDebugging the gccgo debugger support is fairly close to complete, and intellij and/or emacs with debug support might be handy in a pinch.

Longer-term LLDB might be the destination so maybe this is moot.

That said, checking out the source takes a while and there are a lot of devs using Darwin. It might be nice if the gccgo page said, up-front, which platforms were currently supported. Ideally in a manner which pulled from the actual install metadata. The configure might also detect this ASAP. Just saying. When Mr. Kernighan's book is released there will be a wave of people looking into the new-and-improved 'c'.

[ianlancetaylor]

Approximately a dup of #463 . I don't especially want to change the doc/install/gccgo page because it will simply go out of date.

[wrabbit-revisited]

Whoever closes the bug can update the page.