Small tweaks for clang cross compilation cases

[andrewvoznytsa]

• Add --gcc-toolchain to clang flags if sysroot is used (sometimes clang is unable to auto-detect gcc toolchain)
• Add gcc_toolchain option for clang - for the case when you are cross-compiling on Ubuntu for Ubuntu via Ubuntu's multiach support (without use_sysroot=true)
• Add dwarf_version argument for clang - dwarf 4+ is not always compatible with binutils used for target OS
• Set clang's --target for x86/x64 linux builds too - in case of cross-compilation when host and target cpu are the same (x86_64) but OSes are different clang used host tools to compile (in case of x86_64-apple-darwin21.4.0 to x86_64-linux-gnu cross compilation the host's stuff was used)
• Initial win/clang cross-compilation support (macOS/Linux=>Windows). Currently requires //.msvc.toolchain file that should have toolchain data in the same format as toolchain/win/toolchain.py outputs