Restore the stdbool.h meaning of bool after including altivec.h.

FSF GCC's altivec.h does "#define bool __bool" (the vector-bool contextual
keyword), poisoning every subsequent declaration that uses bool, e.g.
SDL_utils_c.h:82 when building SDL_blit_N.c with -maltivec:
  error: incompatible types when returning type 'int' but
  '__vector __bool int' was expected
SDL_blit_N.c only uses the vector/pixel contextual keywords, never vector
bool or vector pixel, so dropping the bool macro is safe. This mirrors what
SDL2's SDL_cpuinfo.h did (#undef bool after altivec.h), adapted for SDL3
where bool must keep working afterwards.

--- include/SDL3/SDL_intrin.h
+++ include/SDL3/SDL_intrin.h
@@ -243,6 +243,16 @@
 #if defined(__ALTIVEC__) && defined(SDL_ENABLE_ALTIVEC)
 #define SDL_ALTIVEC_INTRINSICS 1
 #include <altivec.h>
+/* With FSF GCC, altivec.h redefines bool as the AltiVec vector-bool keyword,
+   which breaks every later declaration using the stdbool.h bool. SDL's
+   AltiVec code only needs the vector/pixel contextual keywords, so restore
+   the standard meaning of bool. */
+#ifndef __APPLE_ALTIVEC__
+#undef bool
+#ifndef __cplusplus
+#define bool _Bool
+#endif
+#endif
 #endif
 #ifndef SDL_DISABLE_NEON
 #  ifdef __ARM_NEON