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 7490471796f287cb284b21ec2643af98e9982b53)
(cherry picked from commit 17e6208c1bd106a29efa16414d1b3dfea7f6e206)

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);