llvm-shlib: Fix mingw dll exports (#148772)
In c87d198cd964f37343083848f8fdd58bb0b00156, the `__jit_debug_*` symbols
gained explicit dllexport annotations. Unfortunately, mingw's linkers
have a quirk where the presence of any dllexport symbols at all will
switch off the `-export-all-symbols` flag, so without a full conversion
to dllexport annotations (#109483), the mingw LLVM dll build is broken
in LLVM 20+ when building with GCC (when building with clang,
LLVM_ALWAYS_EXPORT expands to the default visibility attribute,
see extended discussion in #148772).
Fix this by adding the flag explicitly as was done for
clang-shlib earlier in https://reviews.llvm.org/D151620.
