[release-branch.go1.18] misc/cgo/testcshared: handle unsuffixed dlltool path

Adapt the testcshared tests to handle the case where the path output
by invoking

  gcc -print-prog-name=dlltool

is a path lacking the final ".exe" suffix (this seems to be what clang
is doing); tack it on before using if this is the case.

Updates #57704.
Fixes #57705.

Change-Id: I04fb7b9fc90677880b1ced4a4ad2a8867a3f5f86
Reviewed-on: https://go-review.googlesource.com/c/go/+/451816


TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
(cherry picked from commit 771a98d6)
Reviewed-on: https://go-review.googlesource.com/c/go/+/461176


Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>