Remove DXC_BUILD_ARCH variable

[llvm-beanz]

This commit simplifies the cross-targeting build support to not require
specifying the build architecture explicitly. Instead it is derived from
the build tools used in the generation.

I've tested this change locally for Win32, x64 and arm64.

[AppVeyorBot]

❌ Build DirectXShaderCompiler 1.0.1004 failed (commit 27ada3d352 by @llvm-beanz)

[hekota]

Could you please check if hctbuild -arm64ec works? I suspect it might not in this case. There is no arm64ec or arm64x directory in WinSDK, we should use the arm64 one.

[llvm-beanz]

I'm struggling to get arm64ec to build locally without this change. I'm sure I don't have my system setup correctly :(

[AppVeyorBot]

✅ Build DirectXShaderCompiler 1.0.1007 completed (commit 13192bc023 by @llvm-beanz)

[gongminmin]

CMAKE_<LANG>_COMPILER_ARCHITECTURE_ID is introduced in cmake 3.10. The cmake_minimum_required(VERSION 2.8.12.2) in root cmakelists may need to be changed, too.

[AppVeyorBot]

✅ Build DirectXShaderCompiler 1.0.2230 completed (commit bc0629f9f5 by @llvm-beanz)

[pow2clk]

Looks good. Can you confirm that arm is working as Helena was worried about?

[llvm-beanz]

Arm64 works fine for me locally, but I can't get Arm64EC to build without this change, and with it I get the same failure.

[AppVeyorBot]

✅ Build DirectXShaderCompiler 1.0.2245 completed (commit 2e42d7ef52 by @llvm-beanz)

[hekota]

Go ahead and complete it, the ARM64EC should work down the line.

[llvm-beanz]

This works on arm64ec with #4808, which works around underlying bugs in CMake and COFF object file size limitations for arm64ec.