lib/x86: increase AVX-VNNI gcc prerequisite to 12.1

Although gcc 11.1 supports AVX-VNNI, a popular distro pairs it with a
binutils version that does not.  Require gcc 12 instead.

Resolves #365

lib/x86/adler32_impl.h

@@ -52,8 +52,15 @@
 /*
  * AVX-VNNI implementation.  This is used on CPUs that have AVX2 and AVX-VNNI
  * but don't have AVX-512, for example Intel Alder Lake.
+ *
+ * Unusually for a new CPU feature, gcc added support for the AVX-VNNI
+ * intrinsics (in gcc 11.1) slightly before binutils added support for
+ * assembling AVX-VNNI instructions (in binutils 2.36).  Distros can reasonably
+ * have gcc 11 with binutils 2.35.  Because of this issue, we check for gcc 12
+ * instead of gcc 11.  (libdeflate supports direct compilation without a
+ * configure step, so checking the binutils version is not always an option.)
  */
-#if GCC_PREREQ(11, 1) || CLANG_PREREQ(12, 0, 13000000) || MSVC_PREREQ(1930)
+#if GCC_PREREQ(12, 1) || CLANG_PREREQ(12, 0, 13000000) || MSVC_PREREQ(1930)
 #  define adler32_x86_avx2_vnni	adler32_x86_avx2_vnni
 #  define SUFFIX			   _avx2_vnni
 #  define ATTRIBUTES		_target_attribute("avx2,avxvnni")