Add minimal load-time DLL support on Windows, support dllimport storage class

[HertzDevil]

This patch is required for minimal support of load-time dynamic linking on Windows. For this to work,

• Prepare a directory, distinct from the current Crystal installation's CRYSTAL_LIBRARY_PATH, which we will use to place the .lib and .exp files. Set CRYSTAL_LIBRARY_PATH to point to it;
• Build the minimal dynamic libraries needed by the runtime. Run the following in Powershell: (this is based on the CI job)

  git clone --config core.autocrlf=false -b v8.2.0 https://github.com/ivmai/bdwgc bdwgc
  git clone --config core.autocrlf=false -b v7.6.10 https://github.com/ivmai/libatomic_ops bdwgc\libatomic_ops
  cd bdwgc
  cmake . -DBUILD_SHARED_LIBS=ON -Denable_large_config=ON -DCMAKE_POLICY_DEFAULT_CMP0091=NEW -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreadedDLL -DCMAKE_FIND_USE_SYSTEM_ENVIRONMENT_PATH=OFF
  cmake --build . --config Release
  cd ..
  
  Invoke-WebRequest https://cs.stanford.edu/pub/exim/pcre/pcre-8.43.zip -OutFile pcre.zip
  7z x pcre.zip
  Move-Item pcre-* pcre
  cd pcre
  cmake . -DBUILD_SHARED_LIBS=ON -DPCRE_BUILD_PCREGREP=OFF -DPCRE_BUILD_TESTS=OFF -DPCRE_SUPPORT_UNICODE_PROPERTIES=ON -DPCRE_SUPPORT_JIT=ON -DCMAKE_FIND_USE_SYSTEM_ENVIRONMENT_PATH=OFF
  cmake --build . --config Release
  cd ..

• Copy ./bdwgc/Release/gc.lib and ./pcre/Release/pcre.lib to the CRYSTAL_LIBRARY_PATH we just set. Same for the associated .exp files;
• Build a test program:

  > echo puts ARGV[0].match(/../) > pcretest.cr
  > crystal build pcretest.cr -Dpreview_dll -Dwithout_iconv

• Place gc.dll and pcre.dll in the same directory as the pcretest.exe executable;
• Run it!

  > pcretest 123
  Regex::MatchData("12")

Without this patch, the compilation step would fail with:

_main.obj : error LNK2019: unresolved external symbol pcre_malloc referenced in function __crystal_main
_main.obj : error LNK2019: unresolved external symbol pcre_free referenced in function __crystal_main
G-C-.obj : error LNK2019: unresolved external symbol GC_stackbottom referenced in function .2A.GC.3A..3A.current_thread_stack_bottom.3A.Tuple.28.Pointer.28.Void.29..2C..20.Pointer.28.Void.29..29.

This PR thus adds the dllimport storage class to any external lib variables, through LLVM.

The -Dpreview_dll flag is also added because currently the compiler defaults to static builds on Windows, even without the --static build flag, and everything would immediately break if the current command line suddenly starts using dynamic linking. With a minimal working example we can later sort out the details for dynamic linking on Windows.

[HertzDevil]

The instructions to build PCRE2, which is necessary since Crystal 1.8.0, are:

git clone --config core.autocrlf=false -b pcre2-10.42 https://github.com/PCRE2Project/pcre2.git pcre2
cd pcre2
cmake . -DBUILD_STATIC_LIBS=OFF -DBUILD_SHARED_LIBS=ON -DPCRE2_BUILD_PCRE2GREP=OFF -DPCRE2_BUILD_TESTS=OFF -DPCRE2_SUPPORT_UNICODE=ON -DPCRE2_SUPPORT_JIT=ON -DCMAKE_FIND_USE_SYSTEM_ENVIRONMENT_PATH=OFF
cmake --build . --config Release
cd ..
copy pcre2\Release\pcre2-8.lib .
copy pcre2\Release\pcre2-8.dll .