util: use c11 alignas instead of rolling our own

Due to how alignas is defined, it itsn't allowed to use it on a struct,
it needs to be used on the first member instead. So move the declaration
in those cases.

This still leaves the ALIGN16 macro using compiler-specific directives,
because it's a lot of work to untangle the above. This probably deserves
its own MR.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16908>

src/util/ralloc.c

@@ -41,20 +41,14 @@
  * 64-bit), avoiding performance penalities on x86 and alignment faults on
  * ARM.
  */
-struct
-#ifdef _MSC_VER
-#if _WIN64
-__declspec(align(16))
-#else
- __declspec(align(8))
-#endif
-#elif defined(__LP64__)
- __attribute__((aligned(16)))
-#else
- __attribute__((aligned(8)))
-#endif
-   ralloc_header
+struct ralloc_header
 {
+#if defined(__LP64__) || defined(_WIN64)
+   alignas(16)
+#else
+   alignas(8)
+#endif
+
 #ifndef NDEBUG
    /* A canary value used to determine whether a pointer is ralloc'd. */
    unsigned canary;
@@ -554,15 +548,15 @@ ralloc_vasprintf_rewrite_tail(char **str, size_t *start, const char *fmt,
 #define SUBALLOC_ALIGNMENT 8
 #define LMAGIC 0x87b9c7d3
 
-struct
-#ifdef _MSC_VER
- __declspec(align(8))
-#elif defined(__LP64__)
- __attribute__((aligned(16)))
+struct linear_header {
+
+   /* align first member to align struct */
+#if defined(__LP64__) || defined(_WIN64)
+   alignas(16)
 #else
- __attribute__((aligned(8)))
+   alignas(8)
 #endif
-   linear_header {
+
 #ifndef NDEBUG
    unsigned magic;   /* for debugging */
 #endif