Fix zstd-dll build missing dependencies (#3496)

* Fixes zstd-dll build (https://github.com/facebook/zstd/issues/3492):
- Adds pool.o and threading.o dependency to the zstd-dll target
- Moves custom allocation functions into header to avoid needing to add dependency on common.o
- Adds test target for zstd-dll
- Adds github workflow that buildis zstd-dll

lib/common/zstd_internal.h

@@ -350,11 +350,6 @@ typedef struct {
 const seqStore_t* ZSTD_getSeqStore(const ZSTD_CCtx* ctx);   /* compress & dictBuilder */
 int ZSTD_seqToCodes(const seqStore_t* seqStorePtr);   /* compress, dictBuilder, decodeCorpus (shouldn't get its definition from here) */
 
-/* custom memory allocation functions */
-void* ZSTD_customMalloc(size_t size, ZSTD_customMem customMem);
-void* ZSTD_customCalloc(size_t size, ZSTD_customMem customMem);
-void ZSTD_customFree(void* ptr, ZSTD_customMem customMem);
-
 
 /* ZSTD_invalidateRepCodes() :
  * ensures next compression will not use repcodes from previous block.