misc/cgo: backport needed for dlltool fix [1.18 backport]

[gopherbot]

@thanm requested issue #57704 to be considered for backport to the next 1.18 minor release.

@gopherbot please open backport issues

[gopherbot]

Change https://go.dev/cl/461176 mentions this issue: misc/cgo/testcshared: handle unsuffixed dlltool path

[cagedmantis]

Approved as this is a test only fix.

[gopherbot]

Closed by merging aa0c01f to release-branch.go1.18.