cmd/link, runtime: record size of itabs

We were depending on runtime.etypes immediately following the itabs.
However, on AIX, runtime.etypes, as a zero-sized symbol, can float
when using external linking. It won't necessarily stay right at the
end of the itabs. Rather than worry about this, just record the size
of the itab data.

In practice it almost always works on AIX, but it fails the runtime
test TestSchedPauseMetrics/runtime/debug.WriteHeapDump,
which fails when iterating over all the itabs.

Tested on AIX. This should fix AIX on the build dashboard.

Change-Id: Id3a113b75b93fa8440c047e92f764ab81423df48
Reviewed-on: https://go-review.googlesource.com/c/go/+/760203
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

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