x/build: add a Windows builder with clang

[jayconrod]

In theory, we should be able to use either GCC or Clang on all platforms where cgo is supported. Currently, none of the Windows builders use Clang. It would be good to verify cgo works in that configuration.

[gopherbot]

Change https://golang.org/cl/291630 mentions this issue: cmd/link: set SizeOfRawData filed in COFF files for .bss section

[gopherbot]

Change https://golang.org/cl/291630 mentions this issue: cmd/link: set SizeOfRawData field in COFF files for .bss section

[gopherbot]

Change https://golang.org/cl/291632 mentions this issue: cmd/link: do not pass -Bsymbolic for PE DLLs

[gopherbot]

Change https://golang.org/cl/291633 mentions this issue: cmd/link: set .ctors COFF section to writable and aligned to 8 bytes

[gopherbot]

Change https://golang.org/cl/291633 mentions this issue: cmd/link: set .ctors COFF section to writable and aligned

[zx2c4]

@jayconrod git master now has patches to make this work.

Here's a toolchain you can use: https://github.com/mstorsjo/llvm-mingw/releases/download/20201020/llvm-mingw-20201020-msvcrt-x86_64.zip

I've had good luck with this, and at some point we might consider encouraging the llvm toolchains, since only those support arm.