configure: Check for the AArch64 CRC extension

Name the feature "arm_crc" rather than plain "crc", to make it
clear that this is about a CPU feature extension, not CRC
implementations in general.

This requires dealing with the extension slightly differently
than other extensions, as the name of the feature and the
".arch_extension" extension name differ.

Naming it with an "arm" prefix rather than "aarch64", as the
CPU extension also is available in 32 bit ARM form, even though
we don't intend to use it there.

configure

@@ -476,6 +476,7 @@ Optimization options (experts only):
   --disable-armv6t2        disable armv6t2 optimizations
   --disable-vfp            disable VFP optimizations
   --disable-neon           disable NEON optimizations
+  --disable-arm-crc        disable ARM/AArch64 CRC optimizations
   --disable-dotprod        disable DOTPROD optimizations
   --disable-i8mm           disable I8MM optimizations
   --disable-sve            disable SVE optimizations
@@ -2231,6 +2232,7 @@ ARCH_EXT_LIST_ARM="
     armv6
     armv6t2
     armv8
+    arm_crc
     dotprod
     i8mm
     neon
@@ -2504,6 +2506,7 @@ SYSTEM_LIBRARIES="
 
 TOOLCHAIN_FEATURES="
     as_arch_directive
+    as_archext_crc_directive
     as_archext_dotprod_directive
     as_archext_i8mm_directive
     as_archext_sve_directive
@@ -2838,6 +2841,7 @@ intrinsics_sse2_deps="sse2"
 vfp_deps="arm"
 vfpv3_deps="vfp"
 setend_deps="arm"
+arm_crc_deps="aarch64"
 dotprod_deps="aarch64 neon"
 i8mm_deps="aarch64 neon"
 sve_deps="aarch64 neon"
@@ -6470,7 +6474,8 @@ if enabled aarch64; then
     # internal assembler in clang 3.3 does not support this instruction
     enabled neon && check_insn neon 'ext   v0.8B, v0.8B, v1.8B, #1'
 
-    archext_list="dotprod i8mm sve sve2 sme"
+    archext_list="arm_crc dotprod i8mm sve sve2 sme"
+    enabled arm_crc && check_archext_name_insn arm_crc crc 'crc32x w0, w0, x0'
     enabled dotprod && check_archext_insn dotprod 'udot v0.4s, v0.16b, v0.16b'
     enabled i8mm    && check_archext_insn i8mm    'usdot v0.4s, v0.16b, v0.16b'
     enabled sve     && check_archext_insn sve     'whilelt p0.s, x0, x1'

libavutil/aarch64/asm.S

@@ -40,6 +40,14 @@
         .arch           AS_ARCH_LEVEL
 #endif
 
+#if HAVE_AS_ARCHEXT_CRC_DIRECTIVE
+#define ENABLE_ARM_CRC  .arch_extension crc
+#define DISABLE_ARM_CRC .arch_extension nocrc
+#else
+#define ENABLE_ARM_CRC
+#define DISABLE_ARM_CRC
+#endif
+
 #if HAVE_AS_ARCHEXT_DOTPROD_DIRECTIVE
 #define ENABLE_DOTPROD  .arch_extension dotprod
 #define DISABLE_DOTPROD .arch_extension nodotprod
@@ -80,6 +88,7 @@
 #define DISABLE_SME
 #endif
 
+DISABLE_ARM_CRC
 DISABLE_DOTPROD
 DISABLE_I8MM
 DISABLE_SVE