Makes things up-to-date with the upstream at
https://code.videolan.org/videolan/x86inc.asm

Specifying every individual change is difficult as there have been
divergences and cherry-picks over time, but the full upstream change
log can be found at
https://code.videolan.org/videolan/x86inc.asm/-/commits/master?ref_type=heads

Tested and passes FATE on Linux (x86-64), macOS (x86-64), and Windows
(x86-64 and x86-32). The macOS system with AVX2, the rest with
AVX512ICL.

The following local changes compared to upstream remain (is the OS/2
part still relevant?):
@@ -37,2 +37,5 @@

+%if HAVE_ALIGNED_STACK
+    %define STACK_ALIGNMENT 16
+%endif
 %ifndef STACK_ALIGNMENT
@@ -86,4 +89,11 @@

+; aout does not support align=
+; NOTE: This section is out of sync with x264, in order to
+; keep supporting OS/2.
 %macro SECTION_RODATA 0-1 16
-    %ifidn __OUTPUT_FORMAT__,win32
+    %ifidn __OUTPUT_FORMAT__,aout
+        SECTION .text
+    %elifidn __OUTPUT_FORMAT__,coff
+        SECTION .text
+    %elifidn __OUTPUT_FORMAT__,win32
         SECTION .rdata align=%1