cmd/link: support windows/arm

Enable the Go linker to generate executables for windows/arm.

Generates PE relocation tables, which are used by Windows to
dynamically relocate the Go binary in memory. Windows on ARM
requires all modules to be relocatable, unlike x86/amd64 which are
permitted to have fixed base addresses.

Updates #26148

Change-Id: Ie63964ff52c2377e121b2885e9d05ec3ed8dc1cd
Reviewed-on: https://go-review.googlesource.com/125648


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>