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attributes: Don't align to more than what assembly needs/benefits from

For arm/arm64, there's no need to align any buffer to 32 bytes
as the assembly doesn't need it and doesn't benefit from it.

This would be much more elegant if defined like this:

    #define MAX_ALIGN 16
    #define ALIGN(align) __attribute__((aligned(MIN(align, MAX_ALIGN))))

This works for GCC and Clang, but the MSVC alignment __declspec
needs a literal alignment value, it can't handle an expression.

--- a/include/common/attributes.h

+++ b/include/common/attributes.h

@@ -28,8 +28,23 @@

 #ifndef __DAV1D_COMMON_ATTRIBUTES_H__

 #define __DAV1D_COMMON_ATTRIBUTES_H__

+#include "config.h"

+

 #include <stddef.h>

+#if ARCH_X86

+#define ALIGN_32_VAL 32

+#define ALIGN_16_VAL 16

+#elif ARCH_ARM || ARCH_AARCH64

+// ARM doesn't benefit from anything more than 16 byte alignment.

+#define ALIGN_32_VAL 16

+#define ALIGN_16_VAL 16

+#else

+// No need for extra alignment on platforms without assembly.

+#define ALIGN_32_VAL 8

+#define ALIGN_16_VAL 8

+#endif

+

 /*

  * API for variables, struct members (ALIGN()) like:

  * uint8_t var[1][2][3][4]

@@ -51,10 +66,10 @@

  * ALIGN_STK_$align(uint8_t, var, 1, [2][3][4])

  */

 #define ALIGN_STK_32(type, var, sz1d, sznd) \

-    ALIGN(type var[sz1d]sznd, 32)

+    ALIGN(type var[sz1d]sznd, ALIGN_32_VAL)

 // as long as stack is itself 16-byte aligned, this works (win64, gcc)

 #define ALIGN_STK_16(type, var, sz1d, sznd) \

-    ALIGN(type var[sz1d]sznd, 16)

+    ALIGN(type var[sz1d]sznd, ALIGN_16_VAL)

 /*

  * Forbid inlining of a function: