cmd/link: read crt2.o for windows internal-linking CGO

For Windows internal linking with CGO, when using more modern
LLVM-based compilers, we may need to read in the object file "crt2.o"
so as to have a definition of "atexit" (for example when linking the
runtime/cgo test), and we also need to allow for the possibility that
a given host archive might have to be looked at more than once. The goal
here is to get all.bash working on Windows when using an up to date
mingw C compiler (including those based on clang + LLD).

This patch also adds a new "hostObject" helper routine, similar to
"hostArchive" but specific to individual object files. There is also a
change to hostArchive to modify the pseudo-package name assigned when
reading archive elements: up until this point, a package name of
"libgcc" was used (even when reading a host archive like
"libmingex.a"), which led to very confusing errors messages if symbols
were missing or there were duplicate definitions.

Updates #35006.

Change-Id: I19c17dea9cfffa9e79030fc23064c7c63a612097
Reviewed-on: https://go-review.googlesource.com/c/go/+/382838
Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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