amd: Win32 fixes
Stub sections that don't have _WIN32 support.
Replace vasprintf with vfprintf.
Work around VC++ 65535 string limit for string tables.
Cast to int for %d snprintf argument.
Use uint32_t for bit fields to fix negative indexing on MSVC.
Modify ac_init_llvm_target signature to match callback.
Modify ac_init_shared_llvm_once declarations to match.
Splitting off changes from !6162 (merged).