cmd/compile: use file content, not suffix, to distinguish .a and .o files

cmd/compile: use file content, not suffix, to distinguish .a and .o files

This allows reading from package storage systems that may not
preserve the .a suffix (used with -importcfg).

Fixes #20579 (combined with CLs earlier in stack).

Change-Id: If2fc6a3d01bd0170a757e1f2ba9a22a4d9be7dbf
Reviewed-on: https://go-review.googlesource.com/44853
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>