misc/cgo: generate Windows import libraries for clang

LLD won't import a .dll directly and instead requires an import library.
So generate these using -out-implib, the same way as was done in CL
312046, where it makes sense, and elsewhere build the import library
using a def file. We can't use -out-implib all the time, because the
output file gets overwritten each time the linker is called, rather than
merged.

Updates #46502.

Change-Id: Iefe54cb6c576004b83b1039ba673881b8640423d
Reviewed-on: https://go-review.googlesource.com/c/go/+/327211


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>