cmd/link: set runtime.text to the address of the first function

In CL 240065 we changed it to set to FlagTextAddr. Normally it
is the address of the first function, except on plan9/amd64
where, as FlagTextAddr is not aligned, it is rounded up. Set it
to the actual text start address.

Fixes #41137.

Change-Id: I1bba67f5eb4e24d9f745a11350fc999ff48bff45
Reviewed-on: https://go-review.googlesource.com/c/go/+/261644


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>