libgo/configure: remove -fno-section-anchors for AIX

This option is no longer needed. There is no crash without it since
at least gcc-9.

Change-Id: I1f55cf8cbff3ce00eed2d701f4c599381d0936fa
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/260157
Trust: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>