configure: Improve aarch64 feature detection on older, broken Clang versions

Clang versions before 17 (Xcode versions up to and including 15.0)
had a very annoying bug in its behaviour of the ".arch" directive
in assembly. If the directive only contained a level, such as
".arch armv8.2-a", it did validate the name of the level, but it
didn't apply the level to what instructions are allowed. The level
was applied if the directive contained an extra feature enabled,
such as ".arch armv8.2-a+crc" though. It was also applied on the
next ".arch_extension" directive.

This bug, combined with the fact that the same versions of Clang
didn't support the dotprod/i8mm extension names in either
".arch <level>+<feature>" or in ".arch_extension", could lead to
unexepcted build failures.

As the dotprod/i8mm extensions couldn't be enabled dynamically
via the ".arch_extension" directive, someone building ffmpeg could
try to enable them by configuring their build with
--extra-cflags="-march=armv8.6-a".

During configure, we test for support for the i8mm instructions
like this:

    # Built with -march=armv8.6-a
    .arch armv8.2-a             # Has no visible effect here
    #.arch_extension i8mm       # Omitted as the extension name isn't known
    usdot v0.4s, v0.16b, v0.16b
    # Successfully assembled as armv8.6-a is the effective level,
    # and i8mm is enabled implicitly in armv8.6-a.

Thus, we would enable assembling those instructions. However if
we later check for another extension, such as sve (which those
versions of Clang actually do support), we can later run into the
following situation when building actual code:

    # Built with -march=armv8.6-a
    .arch armv8.2-a             # Has no visible effect here
    #.arch_extension i8mm       # Omitted as the extension name isn't known
    .arch_extension sve         # Included as "sve" is as supported extension name
    # .arch_extension effectively activates the previous .arch directive,
    # so the effective level is armv8.2-a+sve now.
    usdot v0.4s, v0.16b, v0.16b
    # Fails to build the instructions that require i8mm. Despite the
    # configure check, the unrelated ".arch_extension sve" directive
    # breaks the functionality of the i8mm feature.

This patch avoids this situation:
- By adding a dummy feature such as "+crc" on the .arch directive
  (if supported), we make sure that it does get applied immediately,
  avoiding it taking effect spuriously at a later unrelated
  ".arch_extension" directive.
- By checking for higher arch levels such as armv8.4-a and armv8.6-a,
  we can assemble the dotprod and i8mm extensions without the user
  needing to pass -march=armv8.6-a. This allows using the dotprod/i8mm
  codepaths via runtime detection while keeping the binary runnable
  on older versions. I.e. this enables the i8mm codepaths on Apple M2
  machines while built with Xcode's Clang.

TL;DR: Enable the I8MM extensions for Apple M2 without the user needing
to do a custom configuration; avoid potential build breakage if a user
does such a custom configuration.

Once Xcode versions that have these issues fixed are prevalent, we
can consider reverting this change.

Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -6045,7 +6045,26 @@ check_inline_asm inline_asm_nonlocal_labels '"Label:\n"'
 if enabled aarch64; then
     as_arch_level="armv8-a"
     check_as as_arch_directive ".arch $as_arch_level"
-    enabled as_arch_directive && check_arch_level armv8.2-a
+    if enabled as_arch_directive; then
+        # Check for higher .arch levels. We only need armv8.2-a in order to
+        # enable the extensions we want below - we primarily want to control
+        # them via .arch_extension. However:
+        #
+        # Clang before version 17 (Xcode versions up to and including 15.0)
+        # didn't support controlling the dotprod/i8mm extensions via
+        # .arch_extension; thus try to enable them via the .arch level as well.
+        for level in armv8.2-a armv8.4-a armv8.6-a; do
+            check_arch_level $level
+        done
+        # Clang before version 17 (Xcode versions up to and including 15.0)
+        # also had a bug (https://github.com/llvm/llvm-project/issues/32220)
+        # causing a plain ".arch <level>" to not have any effect unless it
+        # had an extra "+<feature>" included - but it was activate on the next
+        # ".arch_extension" directive. Check if we can include "+crc" as dummy
+        # feature to make the .arch directive behave as expected and take
+        # effect right away.
+        check_arch_level "${as_arch_level}+crc"
+    fi
 
     enabled armv8 && check_insn armv8 'prfm   pldl1strm, [x0]'
     # internal assembler in clang 3.3 does not support this instruction