[bazel] Update/fix Windows toolchain

This uses the Clang/MSVC libraries introduced in [1] for the GN
build. Additionally, it uses the dynamic path resolution added
in [2] and avoids the need for brittle trampolines from [3]

With this, I can run `bazelisk build //:core` on a Windows machine
w/o any special setup.

The defines that are always set were copied from GN [4]

[1] https://review.skia.org/1245496
[2] https://review.skia.org/1287856
[3] https://review.skia.org/1293456
[4] https://github.com/google/skia/blob/f5778d05acb3bb6eb08ed25bc4db65c14f383108/gn/skia/BUILD.gn#L95-L100

Change-Id: I92f4500f95f22eb9b4d933c9a60426f4b36f3190
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1287416
Reviewed-by: Eric Boren <borenet@google.com>