runtime: remove use of -mnop-fun-dllimport from cgotest win.go
runtime: remove use of -mnop-fun-dllimport from cgotest win.go

This flag is not supported by clang, so remove it from the cgo cflags
when building for windows. It is clear that it was needed at some
point in the past, but it doesn't appear to be needed at the moment,
since all.bash passes on windows without it now.

Updates #35006.

Change-Id: Ib06c891f516654138e3363e06645cd187e46ce4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/383838
Trust: Alex Brainman <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>
Reviewed-by: Alex Brainman <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>
Trust: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Run-TryBot: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,golang/go,golang/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,golang/go,golang/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go