[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.

contrib/pzstd/Makefile

@@ -57,19 +57,6 @@ LD_COMMAND  = $(CXX) $^          $(ALL_LDFLAGS) $(LIBS) -pthread -o $@
 CC_COMMAND  = $(CC)  $(DEPFLAGS) $(ALL_CFLAGS)   -c $<  -o $@
 CXX_COMMAND = $(CXX) $(DEPFLAGS) $(ALL_CXXFLAGS) -c $<  -o $@
 
-# Get a list of all zstd files so we rebuild the static library when we need to
-ZSTDCOMMON_FILES := $(wildcard $(ZSTDDIR)/common/*.c) \
-                    $(wildcard $(ZSTDDIR)/common/*.h)
-ZSTDCOMP_FILES   := $(wildcard $(ZSTDDIR)/compress/*.c) \
-                    $(wildcard $(ZSTDDIR)/compress/*.h)
-ZSTDDECOMP_FILES := $(wildcard $(ZSTDDIR)/decompress/*.c) \
-                    $(wildcard $(ZSTDDIR)/decompress/*.h)
-ZSTDPROG_FILES   := $(wildcard $(PROGDIR)/*.c) \
-                    $(wildcard $(PROGDIR)/*.h)
-ZSTD_FILES       := $(wildcard $(ZSTDDIR)/*.h) \
-                    $(ZSTDDECOMP_FILES) $(ZSTDCOMMON_FILES) $(ZSTDCOMP_FILES) \
-                    $(ZSTDPROG_FILES)
-
 # List all the pzstd source files so we can determine their dependencies
 PZSTD_SRCS  := $(wildcard *.cpp)
 PZSTD_TESTS := $(wildcard test/*.cpp)
@@ -189,7 +176,8 @@ roundtrip: test/RoundTripTest$(EXT)
 
 # Use the static library that zstd builds for simplicity and
 # so we get the compiler options correct
-$(ZSTDDIR)/libzstd.a: $(ZSTD_FILES)
+.PHONY: $(ZSTDDIR)/libzstd.a
+$(ZSTDDIR)/libzstd.a:
 	CFLAGS="$(ALL_CFLAGS)" LDFLAGS="$(ALL_LDFLAGS)" $(MAKE) -C $(ZSTDDIR) libzstd.a
 
 # Rules to build the tests