[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.

.circleci/config.yml

@@ -7,6 +7,8 @@ jobs:
     #       preinstalled to reduce installation time.
     docker:
       - image: fbopensource/zstd-circleci-primary:0.0.1
+    # TODO: Re-enable aarch64 build:
+    #       make aarch64build     && make clean
     steps:
       - checkout
       - run:
@@ -14,12 +16,11 @@ jobs:
           command: |
             ./tests/test-license.py
             cc -v; CFLAGS="-O0 -Werror -pedantic" make all && make clean
-            make c99build         ; make clean
-            make c11build         ; make clean
-            make aarch64build     ; make clean
-            make -j regressiontest; make clean
-            make shortest         ; make clean
-            make cxxtest          ; make clean
+            make c99build         && make clean
+            make c11build         && make clean
+            make -j regressiontest&& make clean
+            make shortest         && make clean
+            make cxxtest          && make clean
   # the second half of the jobs are in this test
   short-tests-1:
     docker: