Fix build on PowerPC with uClibc

PPC_FEATURE_HAS_ALTIVEC is not defined by all C libraries. The following
error was raised while testing SDL3 in Buildroot with a PowerPC toolchain
using uClibc-ng:

  src/cpuinfo/SDL_cpuinfo.c: In function 'SDL_GetAltiVecFeatures':
  src/cpuinfo/SDL_cpuinfo.c:351:37: error: 'PPC_FEATURE_HAS_ALTIVEC' undeclared (first use in this function)
    351 |     altivec = getauxval(AT_HWCAP) & PPC_FEATURE_HAS_ALTIVEC;
        |                                     ^~~~~~~~~~~~~~~~~~~~~~~

Provide a fallback definition when it is not defined.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>

src/cpuinfo/SDL_cpuinfo.c

@@ -80,6 +80,10 @@
 #include <sys/auxv.h>
 #endif
 
+#ifndef PPC_FEATURE_HAS_ALTIVEC
+#define PPC_FEATURE_HAS_ALTIVEC 0x10000000
+#endif
+
 #ifdef SDL_PLATFORM_RISCOS
 #include <kernel.h>
 #include <swis.h>