cmd/link: support .def file with MSVC clang toolchain

cmd/link: support .def file with MSVC clang toolchain

lld-link supports .def file, but requires a "-def:" (or "/def:")
flag. (MinGW linker, on the other hand, requires no flag.) Pass
the flag when using MSVC-based toolchain.

CL originally authored by Chressie Himpel.

Change-Id: I8c327ab48d36b0bcbb1d127cff544ffdb06be38e
Reviewed-on: https://go-review.googlesource.com/c/go/+/708716
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Chressie Himpel <chressie@google.com>