WIP: Support using clang in mingw64 environment on Windows

[Tyler-IN]

Primary goal:

• Build and run JSC built with Clang under windows

• Build and test JSC built with Clang and thin-LTO successfully under windows

Other stuff:

• Support building without Udis86 / Disassembler

• Support building release with (some) asserts

• Use UASM instead of MASM

• Drop UASM and MASM mostly or entirely if possible

• Fix CPU guessing under Clang on Windows

• Support building directly from sources instead of "unified sources"

Current CMake magic required to build, to be integrated into CMakeLists or other:

-G "Ninja"
-DRUBY_LIBRARY=/usr/bin/msys-ruby270.dll
-DCMAKE_BUILD_TYPE=Debug
-DINCLUDE_DIRECTORIES:STRING=/usr/include
-DCMAKE_CXX_FLAGS="-g -Og -flto=thin -fuse-ld=lld -fno-rtti -fno-exceptions -stdlib=libc++ -Wno-nonportable-include-path -Wno-ignored-attributes -Wno-implicit-int-float-conversion -UNDEBUG -D_DEBUG"
-DCMAKE_AR="C:\ProgramData\msys64\mingw64\bin\llvm-ar.exe"
-DCMAKE_RANLIB="C:\ProgramData\msys64\mingw64\bin\llvm-ranlib.exe"
-DCMAKE_LINKER="C:\ProgramData\msys64\mingw64\bin\ld.lld.exe --threads"
-DCMAKE_C_LINK_EXECUTABLE="C:\ProgramData\msys64\mingw64\bin\ld.lld.exe --threads"
-DCMAKE_CXX_LINK_EXECUTABLE="C:\ProgramData\msys64\mingw64\bin\ld.lld.exe --threads"
-DCMAKE_EXE_LINKER_FLAGS="-lc++ -lc++abi -lstdc++"
-DCMAKE_SHARED_LINKER_FLAGS="-lc++ -lc++abi -lstdc++"
-DCMAKE_ASM_MASM_COMPILER=uasm

TODO: Look into using CMAKE_SYSROOT, CMAKE_TOOLCHAIN_FILE

This is using Windows CMake instead of MSys/MinGW64 Cmake.

Also needs;

#ifdef __clang__
#define debug_break __builtin_debugtrap
#else
#include <Ultralight/private/util/DebugBreak.h> // or add it inside here
#endif

[Janrupf]

add_definitions is outdated and has been replaced with newer commands. Maybe it can be replaced here then, see
https://cmake.org/cmake/help/latest/command/add_definitions.html for more information.

[Tyler-IN]

Sure, add_compile_options, but add_definitions is used all over the place. Probably do a mass replace as a separate PR.

[Janrupf]

Any reason this is required? This should probably be off, as it allows compiler specific extensions to the language and can cause confusing errors.

[Tyler-IN]

No, I think this was an error

[Tyler-IN]

From trying to build again, looks like this wasn't an error.

[Janrupf]

Is there a reason this check is not a WIN32 check? (Same goes for the following ones)

[Tyler-IN]

I think CMAKE_SYSTEM_NAME is for target, not for host.
Don't want to target Windows under Linux or Mac and assume these are available.

[Janrupf]

WIN32 is set to TRUE when targeting a Windows (like) system, so if you compile with MinGW under Linux, CMake will define WIN32.

It is also a bit weird that they have to be guarded at all. WEBKIT_CHECK_HAVE_INCLUDE checks if the include exists, and if it doesn't, it does not fail the compilation, but just sets the first argument of the macro to false. So even if those checks would be executed on every system, it should under no circumstances break the build.

[TYoung86]

The case I want to avoid is if they return true under Windows, not if they return false.