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]

:x: Build DirectXShaderCompiler 1.0.1004 failed (commit https://github.com/microsoft/DirectXShaderCompiler/commit/27ada3d352 by @llvm-beanz)

[AppVeyorBot]

:white_check_mark: Build DirectXShaderCompiler 1.0.1007 completed (commit https://github.com/microsoft/DirectXShaderCompiler/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]

:white_check_mark: Build DirectXShaderCompiler 1.0.2230 completed (commit https://github.com/microsoft/DirectXShaderCompiler/commit/bc0629f9f5 by @llvm-beanz)

[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]

:white_check_mark: Build DirectXShaderCompiler 1.0.2245 completed (commit https://github.com/microsoft/DirectXShaderCompiler/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.