misc/cgo/testcshared: handle unsuffixed dlltool path

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 #35006.
Updates #53540.

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>