cmd/link,runtime: make textsectmap fields more convenient for runtime

cmd/link,runtime: make textsectmap fields more convenient for runtime

They're only used in a single place.
Instead of calculating the end every time,
calculate it in the linker.

It'd be nice to recalculate baseaddr-vaddr,
but that generates relocations that are too large.

While we're here, remove some pointless uintptr -> uintptr conversions.

Change-Id: I91758f9bff11b365bc3a63fee172dbdc3d90b966
Reviewed-on: https://go-review.googlesource.com/c/go/+/354089
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>