[build] Add support for ASM files in Make + CMake

* Extract out common portion of `lib/Makefile` into `lib/libzstd.mk`.
  Most relevantly, the way we find library files.
* Use `lib/libzstd.mk` in the other Makefiles instead of repeating the
  same code.
* Add a test `tests/test-variants.sh` that checks that the builds of
  `make -C programs allVariants` are correct, and run it in Actions.
* Adds support for ASM files in the CMake build.

The Meson build is not updated because it lists every file in zstd,
and supports ASM off the bat, so the Huffman ASM commit will just add
the ASM file to the list.

The Visual Studios build is not updated because I'm not adding ASM
support to Visual Studios yet.

lib/compress/huf_compress.c

@@ -445,7 +445,7 @@ typedef struct {
 /* RANK_POSITION_DISTINCT_COUNT_CUTOFF == Cutoff point in HUF_sort() buckets for which we use log2 bucketing.
  * Strategy is to use as many buckets as possible for representing distinct
  * counts while using the remainder to represent all "large" counts.
- * 
+ *
  * To satisfy this requirement for 192 buckets, we can do the following:
  * Let buckets 0-166 represent distinct counts of [0, 166]
  * Let buckets 166 to 192 represent all remaining counts up to RANK_POSITION_MAX_COUNT_LOG using log2 bucketing.
@@ -517,7 +517,7 @@ static int HUF_quickSortPartition(nodeElt arr[], int const low, int const high)
 }
 
 /* Classic quicksort by descending with partially iterative calls
- * to reduce worst case callstack size. 
+ * to reduce worst case callstack size.
  */
 static void HUF_simpleQuickSort(nodeElt arr[], int low, int high) {
     int const kInsertionSortThreshold = 8;
@@ -813,6 +813,7 @@ FORCE_INLINE_TEMPLATE void HUF_addBits(HUF_CStream_t* bitC, HUF_CElt elt, int id
         assert(((elt >> dirtyBits) << (dirtyBits + nbBits)) == 0);
         /* We didn't overwrite any bits in the bit container. */
         assert(!kFast || (bitC->bitPos[idx] & 0xFF) <= HUF_BITS_IN_CONTAINER);
+        (void)dirtyBits;
     }
 #endif
 }