Make Makefile work on MSYS2

[HertzDevil]

Resolves part of #6170.

• Uses llvm-config instead of llvm_VERSION for the x86_64-windows-gnu target. Also invokes the find-llvm-config script using sh explicitly and ensures the script returns a Windows path. (This pretty much means a MinGW-w64-based compiler will require the entire MSYS2 environment to work.)
• Increases the stack size from the default 2 MiB to 8 MiB, consistent with builds using the MSVC toolchain.
• Makefile and the bin/crystal script now recognize .build/crystal.exe as the local compiler under MSYS2.
• If .build/crystal.exe already exists and we are on Windows, Makefile now builds a temporary executable first, just like in Makefile.win.
• Ensures Makefile doesn't interpret backslashes in llvm-config.exe's path as escape sequences.
• Adds a separate install_dlls target for installing the compiler's dependent DLLs to the given prefix's bin directory. This is only needed if the installation is to be distributed outside MSYS2.
• Detects the presence of lld which can be installed using the mingw-w64-ucrt-x86_64-lld package.

With this patch, cross-compiling from MSVC-based Crystal will no longer work until #15091 is merged. Instead it can be done from Linux, including WSL, assuming the host and target LLVM versions are identical (right now MSYS2 has 18.1.8):

# on Linux
make clean crystal && make -B target=x86_64-windows-gnu

# on MSYS2's UCRT64 environment
pacman -Sy \
  mingw-w64-ucrt-x86_64-gcc mingw-w64-ucrt-x86_64-pkgconf \
  mingw-w64-ucrt-x86_64-gc mingw-w64-ucrt-x86_64-pcre2 mingw-w64-ucrt-x86_64-libiconv \
  mingw-w64-ucrt-x86_64-zlib mingw-w64-ucrt-x86_64-llvm
cc .build/crystal.obj -o .build/crystal.exe \
  $(pkg-config bdw-gc libpcre2-8 iconv zlib --libs) \
  $(llvm-config --libs --system-libs --ldflags) \
  -lDbgHelp -lole32 -lWS2_32 -Wl,--stack,0x800000