CMakeLists.txt: use GAS by default if building with Clang-CL

[erikbs]

if (MSVC) evaluates to true if building with Clang-CL, but at least if cross-compiling with Clang-CL from a non-Windows host, MASM (default for MSVC) is not understood. Using GAS, however, works perfectly. I therefore propose to restrict this check to trigger only with MSVC and not Clang-CL. I have not tested this extensively, but it works in my case where I use Clang-CL on a GNU/Linux host targeting Windows.

Of course another option is to override the detected assembler using -DBOOST_CONTEXT_ASSEMBLER="gas", but automatic detection makes build scripts a bit cleaner.

[microcai]

cross-compiling hates automatic detection.

[erikbs]

It is relevant when building with LLVM/Clang-CL on Windows without cross-compiling as well, I tested.

If configuring through the MSVC Command Prompt, Context will detect MASM/ml64.exe and use that (ignoring CMAKE_ASM_COMPILER if I set it to Clang-CL). If using only CMake + Ninja + LLVM/Clang-CL, it will still try to use ml64.exe, but fail to find it. With my patch, it will both respect CMAKE_ASM_COMPILER and use Clang-CL if no assembler is specified explicitly.

In my opinion my proposed patch therefore makes sense even if we ignore cross-compilation. Instead of defaulting to MASM for all Windows builds, this would make Context default to using the assembler associated with the compiler, so MASM when building with MSVC and Clang-CL/GAS when building with Clang-CL. I think that is cleaner.