cmd/link: skip TestFlagW on AIX

cmd/link: skip TestFlagW on AIX

The internal/xcoff can only parse XCOFF with symbol table. This
test creates executables without symbol table. Skip the test.

(It might be possible to make internal/xcoff work with binaries
without symbol table? Leave it for the future.)

Fixes #75618.

Change-Id: I273ffefee5376d987accd5aa48c9473d2b3df055
Reviewed-on: https://go-review.googlesource.com/c/go/+/707095
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>