aarch64: Add support for the dotprod and i8mm extensions

This uses the same configure logic as in ffmpeg and dav1d for these features. (The tools had bugs relating to the asm directives for enabling these extensions until not long ago, so the right procedure for enabling them isn’t entirely straightforward.) Also use the same kinds of macros for enabling/disabling the extensions within asm sources as ffmpeg/dav1d.

requested review from @gramner

mentioned in merge request !171 (merged)

added 5 commits

• ea389303 - configure: Check for .arch and .arch_extension for enabling aarch64 extensions
• 2215a447 - aarch64: Use configure detected directives for enabling SVE/SVE2
• a607106d - configure: Check for the dotprod and i8mm aarch64 extensions
• 11eca939 - aarch64: Add flags for runtime detection of dotprod and i8mm
• 680ca572 - aarch64: Add runtime detection of extensions on Windows and macOS

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added 2 commits

• a2f1c48b - aarch64: Add flags for runtime detection of dotprod and i8mm
• cf2979f9 - aarch64: Add runtime detection of extensions on Windows and macOS

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Ping @gramner or @BugMaster

x264.h probably need X264_BUILD bump after adding new consts.

requested changes

added 2 commits

• 8d4b3584 - aarch64: Add flags for runtime detection of dotprod and i8mm
• 5be1dbaa - aarch64: Add runtime detection of extensions on Windows and macOS

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added 1 commit

• 760b8068 - aarch64: Add runtime detection of extensions on Windows and macOS

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resolved all threads

approved this merge request

added 9 commits

• 760b8068...27d83708 - 2 commits from branch videolan:master
• a0191bd8 - configure: Use as_check for checking for aarch64 features
• 72ce1cde - configure: Use as_check for the main check for whether NEON is supported
• f87ca183 - configure: Check for .arch and .arch_extension for enabling aarch64 extensions
• 87044b21 - aarch64: Use configure detected directives for enabling SVE/SVE2
• fc4012fb - configure: Check for the dotprod and i8mm aarch64 extensions
• 0e48d072 - aarch64: Add flags for runtime detection of dotprod and i8mm
• 570f6c70 - aarch64: Add runtime detection of extensions on Windows and macOS

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merged