ffmpeg PowerPC fixes (#8181)

* ffmpeg: vec_xl intrinsic does not exist on older PowerPC processors

use upstream patch (not yet committed)
closes: https://trac.macports.org/ticket/61001

* ffmpeg: remove no-longer-needed compiler selection

this is now taken care of by base and the
compiler.c_standard setting

multimedia/ffmpeg/Portfile

@@ -90,6 +90,10 @@ depends_lib         port:lame \
 
 patchfiles          patch-libavcodec-audiotoolboxenc.c.diff
 
+# work around vec_xl intrinsic does not exist on older PowerPC processors
+# https://trac.ffmpeg.org/ticket/8750
+patchfiles-append     patch-ffmpeg-powerpc-yuv2rgb-altivec-fix.diff
+
 #
 # enable auto configure of asm optimizations
 # requires Xcode 3.1 or better on Leopard
@@ -105,14 +109,6 @@ if {[lsearch [get_canonical_archs] i386] != -1} {
     compiler.blacklist-append {clang < 422.1.7}
 }
 
-platform darwin powerpc {
-    # https://trac.macports.org/ticket/55124
-    # requires newer compiler than gcc4.2 to build
-    compiler.blacklist
-    compiler.whitelist    macports-gcc-6
-    universal_variant     no
-}
-
 # The old ffmpeg port was GPL-2+ as base and had a no_gpl variant, so this keeps us consistent
 # Also, -gpl2 causes other ports to fail to build due to the missing libpostproc (#35473)
 default_variants-append +gpl2

multimedia/ffmpeg/files/patch-ffmpeg-powerpc-yuv2rgb-altivec-fix.diff

@@ -0,0 +1,20 @@
+--- libswscale/ppc/yuv2rgb_altivec.c.orig	2020-08-22 16:39:38.000000000 -0700
++++ libswscale/ppc/yuv2rgb_altivec.c	2020-08-22 16:46:11.000000000 -0700
+@@ -283,6 +283,17 @@
+  * ------------------------------------------------------------------------------
+  */
+
++
++#if !HAVE_VSX
++static inline vector unsigned char vec_xl(signed long long offset, const ubyte *addr)
++{
++	    const vector unsigned char *v_addr = (const vector unsigned char *) (addr + offset);
++	    vector unsigned char align_perm = vec_lvsl(offset, addr);
++	
++	    return (vector unsigned char) vec_perm(v_addr[0], v_addr[1], align_perm);
++}
++#endif /* !HAVE_VSX */
++
+ #define DEFCSP420_CVT(name, out_pixels)                                       \
+ static int altivec_ ## name(SwsContext *c, const unsigned char **in,          \
+                             int *instrides, int srcSliceY, int srcSliceH,     \