cpuinfo: Use auxv for AltiVec on Linux if possible

The SIGILL handler is not very reliable and can cause crashes.

Linux provides the CPU's AltiVec support status in getauxval.

(cherry picked from commit 7490471)

Author: awilfox

src/cpuinfo/SDL_cpuinfo.c

@@ -127,7 +127,7 @@
 #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__)
+#if defined(SDL_ALTIVEC_BLITTERS) && defined(HAVE_SETJMP) && !defined(__MACOSX__) && !defined(__OpenBSD__) && !defined(__FreeBSD__) && (defined(__LINUX__) && !defined(HAVE_GETAUXVAL))
 /* This is the brute force way of detecting instruction sets...
    the idea is borrowed from the libmpeg2 library - thanks!
  */
@@ -356,6 +356,8 @@ static int CPU_haveAltiVec(void)
     elf_aux_info(AT_HWCAP, &cpufeatures, sizeof(cpufeatures));
     altivec = cpufeatures & PPC_FEATURE_HAS_ALTIVEC;
     return altivec;
+#elif defined(__LINUX__) && defined(__powerpc__) && defined(HAVE_GETAUXVAL)
+    altivec = getauxval(AT_HWCAP) & PPC_FEATURE_HAS_ALTIVEC;
 #elif defined(SDL_ALTIVEC_BLITTERS) && defined(HAVE_SETJMP)
     void (*handler)(int sig);
     handler = signal(SIGILL, illegal_instruction);