cmd/go: add a note in help buildmode for c-archive on AIX

As ld on AIX doesn't keep the same layout in .text section,
-Wl,-bnoobjreoder must be passed to gcc when building a C program with a
Go archive.

Change-Id: I89b584cce43ab5792f315192b073923c10d5690e
Reviewed-on: https://go-review.googlesource.com/c/go/+/170538
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: Clément Chigot

src/cmd/go/alldocs.go

@@ -689,6 +689,9 @@ are:
 	-buildmode=plugin
 		Build the listed main packages, plus all packages that they
 		import, into a Go plugin. Packages not named main are ignored.
+
+On AIX, when linking a C program that uses a Go archive built with
+-buildmode=c-archive, you must pass -Wl,-bnoobjreorder to the C compiler.
 `,
 }