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.