Define illegal_instruction() when it will be actually used

(cherry picked from commit 29d2116)

Author: slouken

src/cpuinfo/SDL_cpuinfo.c

@@ -127,7 +127,11 @@
 #define CPU_CFG2_LSX  (1 << 6)
 #define CPU_CFG2_LASX (1 << 7)
 
-#if defined(SDL_ALTIVEC_BLITTERS) && defined(HAVE_SETJMP) && !defined(__MACOSX__) && !defined(__OpenBSD__) && !defined(__FreeBSD__) && (defined(__LINUX__) && !defined(HAVE_GETAUXVAL))
+#if !defined(SDL_CPUINFO_DISABLED) && \
+    !((defined(__MACOSX__) && (defined(__ppc__) || defined(__ppc64__))) || (defined(__OpenBSD__) && defined(__powerpc__))) && \
+    !(defined(__FreeBSD__) && defined(__powerpc__)) && \
+    !(defined(__LINUX__) && defined(__powerpc__) && defined(HAVE_GETAUXVAL)) && \
+    defined(SDL_ALTIVEC_BLITTERS) && defined(HAVE_SETJMP)
 /* This is the brute force way of detecting instruction sets...
    the idea is borrowed from the libmpeg2 library - thanks!
  */