Flatten ZSTD_row_getMatchMask (#2681)

* Flatten ZSTD_row_getMatchMask

* Remove the SIMD abstraction layer.
* Add big endian support.
* Align `hashTags` within `tagRow` to a 16-byte boundary. 
* Switch SSE2 to use aligned reads.
* Optimize scalar path using SWAR.
* Optimize neon path for `n == 32`
* Work around minor clang issue for NEON (https://bugs.llvm.org/show_bug.cgi?id=49577)

* replace memcpy with MEM_readST

* silence alignment warnings

* fix neon casts

* Update zstd_lazy.c

* unify simd preprocessor detection (#3)

* remove duplicate asserts

* tweak rotates

* improve endian detection

* add cast

there is a fun little catch-22 with gcc: result from pmovmskb has to be cast to uint32_t to avoid a zero-extension
but must be uint16_t to get gcc to generate a rotate instruction..

* more casts

* fix casts

better work-around for the (bogus) warning: unary minus on unsigned

lib/common/compiler.h

@@ -197,6 +197,22 @@
     #define STATIC_BMI2 0
 #endif
 
+/* compile time determination of SIMD support */
+#if !defined(ZSTD_NO_INTRINSICS)
+#  if defined(__SSE2__) || defined(_M_AMD64) || (defined (_M_IX86) && defined(_M_IX86_FP) && (_M_IX86_FP >= 2))
+#    define ZSTD_ARCH_X86_SSE2
+#  endif
+#  if defined(__ARM_NEON) || defined(_M_ARM64)
+#    define ZSTD_ARCH_ARM_NEON
+#  endif
+#
+#  if defined(ZSTD_ARCH_X86_SSE2)
+#    include <emmintrin.h>
+#  elif defined(ZSTD_ARCH_ARM_NEON)
+#    include <arm_neon.h>
+#  endif
+#endif  
+
 /* compat. with non-clang compilers */
 #ifndef __has_builtin
 #  define __has_builtin(x) 0