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>

1 files changed, 21 insertions, 0 deletions

diff --git a/src/cmd/objdump/testdata/fmthellocgo.go b/src/cmd/objdump/testdata/fmthellocgo.go
new file mode 100644
index 0000000000..6555c3bacf
--- /dev/null
+++ b/src/cmd/objdump/testdata/fmthellocgo.go
@@ -0,0 +1,21 @@
+package main
+
+import "fmt"
+import "C"
+
+func main() {
+	Println("hello, world")
+	if flag {
+//line fmthello.go:999999
+		Println("bad line")
+		for {
+		}
+	}
+}
+
+//go:noinline
+func Println(s string) {
+	fmt.Println(s)
+}
+
+var flag bool