x86: h264: Don't keep data in the redzone across function calls on 64 bit unix

We know that the called function (ff_chroma_inter_body_mmxext)
doesn't touch the redzone, and thus will be kept intact - thus,
this doesn't fix any bug per se.

However, valgrind's memcheck tool intentionally assumes that the
redzone is clobbered on every function call and function return
(see a long comment in valgrind/memcheck/mc_main.c). This avoids
false positives in that tool, at the cost of an extra stack pointer
adjustment.

The other alternative would be a valgrind suppression for this issue,
but that's an extra burden for everybody that wants to run libavcodec
within valgrind.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/x86/h264_deblock.asm

@@ -821,10 +821,10 @@ cglobal deblock_v_chroma_8, 5,6
 ;                          int8_t *tc0)
 ;-----------------------------------------------------------------------------
 cglobal deblock_h_chroma_8, 5,7
-%if UNIX64
-    %define buf0 [rsp-24]
-    %define buf1 [rsp-16]
-%elif WIN64
+%if ARCH_X86_64
+    ; This could use the red zone on 64 bit unix to avoid the stack pointer
+    ; readjustment, but valgrind assumes the red zone is clobbered on
+    ; function calls and returns.
     sub   rsp, 16
     %define buf0 [rsp]
     %define buf1 [rsp+8]
@@ -840,7 +840,7 @@ cglobal deblock_h_chroma_8, 5,7
     movq  m0, buf0
     movq  m3, buf1
     TRANSPOSE8x4B_STORE PASS8ROWS(t5, r0, r1, t6)
-%if WIN64
+%if ARCH_X86_64
     add   rsp, 16
 %endif
     RET