cmd/internal/objfile: use pe.FileHeader.Machine to reliably determine GOARCH

Current peFile.goarch looks for symbols like "_rt0_386_windows" to
determine GOARCH. But "_rt0_386_windows" is not present in executables
built with cgo.

Use pe.FileHeader.Machine instead. This should work with any Windows
executable, not just with Go built executable.

Fixes #39682

Change-Id: Ie0ffce664f4b8b8fed69b2ecc482425b042a38d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/240957


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>