[flang-rt] support `FLANG_RT_ENABLE_SHARED=ON` on windows

[h-vetinari]

#121782 added support for FLANG_RT_ENABLE_SHARED=ON, except on windows, which currently has

llvm-project/flang-rt/CMakeLists.txt

Lines 154 to 157 in fb7135e

	option(FLANG_RT_ENABLE_STATIC "Build Flang-RT as a static library." ON)
	if (WIN32)
	# Windows DLL currently not implemented.
	set(FLANG_RT_ENABLE_SHARED OFF)

Opening this issue for tracking, and for having an easy reference to point to.

[llvmbot]

@llvm/issue-subscribers-flang-runtime

Author: None (h-vetinari)

https://github.com//pull/121782 added support for `FLANG_RT_ENABLE_SHARED=ON`, except on windows, which currently has

llvm-project/flang-rt/CMakeLists.txt

Lines 154 to 157 in fb7135e

	option(FLANG_RT_ENABLE_STATIC "Build Flang-RT as a static library." ON)
	if (WIN32)
	# Windows DLL currently not implemented.
	set(FLANG_RT_ENABLE_SHARED OFF)

Opening this issue for tracking, and for having an easy reference to point to.

[Meinersbur]

For Windows support, will need to add __declspec(dllexport) to visible functions, or (better for ABI stability) a .def file like OpenMP's dllexports